title: Sea

text: Wind blowing over the surface of the sea causes friction at the interface between air and sea. Not only does this cause waves to form but it also makes the surface seawater move in the same direction as the wind. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics. When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as a gyre is formed. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows around Antarctica. These gyres have followed the same routes for millennia, guided by the topography of the land, the wind direction and the Coriolis effect. The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes. Global climate and weather forecasts are powerfully affected by the world ocean, so global climate modelling makes use of ocean circulation models as well as models of other major components such as the atmosphere, land surfaces, aerosols and sea ice. Ocean models make use of a branch of physics, geophysical fluid dynamics, that describes the large-scale flow of fluids such as seawater.


title: Sea

text: Surface currents only affect the top few hundred metres of the sea, but there are also large-scale flows in the ocean depths caused by the movement of deep water masses. A main deep ocean current flows through all the world's oceans and is known as the thermohaline circulation or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. Both these factors make it denser, and the water sinks. From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards the surface and loops back on itself. It takes a thousand years for this circulation pattern to be completed.


title: Sea

text: Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle, a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, create spits and make beaches disappear and water channels silt up. A rip current can occur when water piles up near the shore from advancing waves and is funnelled out to sea through a channel in the seabed. It may occur at a gap in a sandbar or near a man-made structure such as a groyne. These strong currents can have a velocity of per second, can form at different places at different stages of the tide and can carry away unwary bathers. Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea.


title: Sea

text: Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore, also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude.


title: Sea

text: Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the Moon to its previous position relative to an observer. The Moon's mass is some 27 million times smaller than the Sun, but it is 400 times closer to the Earth. Tidal force or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun. A bulge is formed in the ocean at the place where the Earth is closest to the Moon, because it is also where the effect of the Moon's gravity is stronger. On the opposite side of the Earth, the lunar force is at its weakest and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the Earth. The gravitational attraction of the Sun is also working on the seas, but its effect on tides is less powerful than that of the Moon, and when the Sun, Moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides". In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the combined gravitational effect on tides is less causing the lower "neap tides".


title: Sea

text: A storm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low pressure system, can raise the surface of the sea at high tide dramatically.


title: Sea

text: The Earth is composed of a magnetic central core, a mostly liquid mantle and a hard rigid outer shell (or lithosphere), which is composed of the Earth's rocky crust and the deeper mostly solid outer layer of the mantle. On land the crust is known as the continental crust while under the sea it is known as the oceanic crust. The latter is composed of relatively dense basalt and is some five to ten kilometres (three to six miles) thick. The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number of tectonic plates. In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction. Deep trenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and magma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity.


title: Sea

text: The Earth's deepest trench is the Mariana Trench which extends for about across the seabed. It is near the Mariana Islands, a volcanic archipelago in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7 miles) below the surface of the sea.


title: Sea

text: The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore. A beach is the accumulation of sand or shingle on the shore. A headland is a point of land jutting out into the sea and a larger promontory is known as a cape. The indentation of a coastline, especially between two headlands, is a bay, a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf. Coastlines are influenced by a number of factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as the swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.


title: Sea

text: Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents. Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building of breakwaters, seawalls, dykes and levees and other sea defences. For instance, the Thames Barrier is designed to protect London from a storm surge, while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States.


title: Sea

text: The sea plays a part in the water or hydrological cycle, in which water evaporates from the ocean, travels through the atmosphere as vapour, condenses, falls as rain or snow, thereby sustaining life on land, and largely returns to the sea. Even in the Atacama Desert, where little rain ever falls, dense clouds of fog known as the camanchaca blow in from the sea and support plant life.


title: Sea

text: In central Asia and other large land masses, there are endorheic basins which have no outlet to the sea, separated from the ocean by mountains or other natural geologic features that prevent the water draining away. The Caspian Sea is the largest one of these. Its main inflow is from the River Volga, there is no outflow and the evaporation of water makes it saline as dissolved minerals accumulate. The Aral Sea in Kazakhstan and Uzbekistan, and Pyramid Lake in the western United States are further examples of large, inland saline water-bodies without drainage. Some endorheic lakes are less salty, but all are sensitive to variations in the quality of the inflowing water.


title: Sea

text: Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the lithosphere in the amount of carbon they store. The oceans' surface layer holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of the surface layer and it remains there for much longer periods of time. Thermohaline circulation exchanges carbon between these two layers.


title: Sea

text: Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid, carbonate, and bicarbonate:


title: Sea

text: It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon rich layers as dead soft tissue or in shells and bones as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation.


title: Sea

text: The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only the upper layers, the major part of the ocean exists in permanent darkness. As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life. Marine habitats range from surface water to the deepest oceanic trenches, including coral reefs, kelp forests, seagrass meadows, tidepools, muddy, sandy and rocky seabeds, and the open pelagic zone. The organisms living in the sea range from whales 30 metres (100 ft) long to microscopic phytoplankton and zooplankton, fungi, and bacteria. Marine life plays an important part in the carbon cycle as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providing fish for use as food.


title: Sea

text: Life may have originated in the sea and all the major groups of animals are represented there. Scientists differ as to precisely where in the sea life arose: the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea "black smoker" vents, all of which would have provided protection from damaging ultraviolet radiation which was not blocked by the early Earth's atmosphere.


title: Sea

text: Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf. Alternatively, marine habitats can be divided vertically into pelagic (open water), demersal (just above the seabed) and benthic (sea bottom) habitats. A third division is by latitude: from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters.


title: Sea

text: Coral reefs, the so-called "rainforests of the sea", occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are tropical coral reefs such as Australia's Great Barrier Reef, but cold water reefs harbour a wide array of species including corals (only six of which contribute to reef formation).


title: Sea

text: Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton. About 45 percent of the sea's primary production of living material is contributed by diatoms. Much larger algae, commonly known as seaweeds, are important locally; "Sargassum" forms floating drifts, while kelp form seabed forests. Flowering plants in the form of seagrasses grow in "meadows" in sandy shallows, mangroves line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated salt marshes. All of these habitats are able to sequester large quantities of carbon and support a biodiverse range of larger and smaller animal life.


title: Sea

text: Light is only able to penetrate the top so this is the only part of the sea where plants can grow. The surface layers are often deficient in biologically active nitrogen compounds. The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen, its assimilation, nitrification, anammox and denitrification. Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal.


title: Sea

text: There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually. Some vertebrates such as seabirds, seals and sea turtles return to the land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria, fungi, microalgae, protozoa, fish eggs and various larvae.


title: Sea

text: The pelagic zone contains macro- and microfauna and myriad zooplankton which drift with the currents. Most of the smallest organisms are the larvae of fish and marine invertebrates which liberate eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute. The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other nektonic organisms to large squid, sharks, porpoises, dolphins and whales. Some marine creatures make large migrations, either to other regions of the ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day. Ships can introduce or spread invasive species through the discharge of ballast water or the transport of organisms that have accumulated as part of the fouling community on the hulls of vessels.


title: Sea

text: The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and the seabed provides a range of habitats on or under the surface of the substrate which are used by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home to barnacles, molluscs and crustaceans. The neritic zone has many organisms that need light to flourish. Here, among algal encrusted rocks live sponges, echinoderms, polychaete worms, sea anemones and other invertebrates. Corals often contain photosynthetic symbionts and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which are an important feature of the seabed. These provide a biodiverse habitat for reef dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes around seamounts that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed live demersal fish that feed largely on pelagic organisms or benthic invertebrates. Exploration of the deep sea by submersibles revealed a new world of creatures living on the seabed that scientists had not previously known to exist. Some like the detrivores rely on organic material falling to the ocean floor. Others cluster round deep sea hydrothermal vents where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often found nowhere else. A dead whale sinking to the bottom of the ocean provides food for an assembly of organisms which similarly rely largely on the actions of sulphur-reducing bacteria. Such places support unique biomes where many new microbes and other lifeforms have been discovered.


title: Sea

text: Humans have travelled the seas since they first built sea-going craft. Mesopotamians were using bitumen to caulk their reed boats and, a little later, masted sails. By c. 3000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia. Subsequently, the Austronesian "Lapita" peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji, Tonga, and Samoa. Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes, and in the process they found many new islands, including Hawaii, Easter Island (Rapa Nui), and New Zealand.


title: Sea

text: The Ancient Egyptians and Phoenicians explored the Mediterranean and Red Sea with the Egyptian Hannu reaching the Arabian Peninsula and the African Coast around 2750 BC. In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the Black Sea. Around 500 BC, the Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that reached at least Senegal and possibly Mount Cameroon. In the early Mediaeval period, the Vikings crossed the North Atlantic and even reached the northeastern fringes of North America. Novgorodians had also been sailing the White Sea since the 13th century or before. Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders. The Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in the early fifteenth century, sailing the Indian and Pacific Oceans. In the late fifteenth century, Western European mariners started making longer voyages of exploration in search of trade. Bartolomeu Dias rounded the Cape of Good Hope in 1487 and Vasco da Gama reached India via the Cape in 1498. Christopher Columbus sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan by the novel means of travelling westwards. He made landfall instead on an island in the Caribbean Sea and a few years later, the Venetian navigator John Cabot reached Newfoundland. The Italian Amerigo Vespucci, after whom America was named, explored the South American coastline in voyages made between 1497 and 1502, discovering the mouth of the Amazon River. In 1519 the Portuguese navigator Ferdinand Magellan led the Spanish Magellan-Elcano expedition which would be the first to sail around the world.


title: Sea

text: As for the history of navigational instrument, a compass was first used by the ancient Greeks and Chinese to show where north lies and the direction in which the ship is heading. The latitude (an angle which ranges from 0° at the equator to 90° at the poles) was determined by measuring the angle between the Sun, Moon or a specific star and the horizon by the use of an astrolabe, Jacob's staff or sextant. The longitude (a line on the globe joining the two poles) could only be calculated with an accurate chronometer to show the exact time difference between the ship and a fixed point such as the Greenwich Meridian. In 1759, John Harrison, a clockmaker, designed such an instrument and James Cook used it in his voyages of exploration. Nowadays, the Global Positioning System (GPS) using over thirty satellites enables accurate navigation worldwide.


title: Sea

text: With regards to maps that are vital for navigation, in the second century, Ptolemy mapped the whole known world from the "Fortunatae Insulae", Cape Verde or Canary Islands, eastward to the Gulf of Thailand. This map was used in 1492 when Christopher Columbus set out on his voyages of discovery. Subsequently, Gerardus Mercator made a practical map of the world in 1538, his map projection conveniently making rhumb lines straight. By the eighteenth century better maps had been made and part of the objective of James Cook on his voyages was to further map the ocean. Scientific study has continued with the depth recordings of the "Tuscarora", the oceanic research of the Challenger voyages (1872–1876), the work of the Scandinavian seamen Roald Amundsen and Fridtjof Nansen, the Michael Sars expedition in 1910, the German Meteor expedition of 1925, the Antarctic survey work of "Discovery II" in 1932, and others since. Furthermore, in 1921, the International Hydrographic Organization (IHO) was set up, and it constitutes the world authority on hydrographic surveying and nautical charting. A fourth edition draft was published in 1986 but so far several naming disputes (such as the one over the Sea of Japan) have prevented its ratification.


title: Sea

text: Scientific oceanography began with the voyages of Captain James Cook from 1768 to 1779, describing the Pacific with unprecedented precision from 71 degrees South to 71 degrees North. John Harrison's chronometers supported Cook's accurate navigation and charting on two of these voyages, permanently improving the standard attainable for subsequent work. Other expeditions followed in the nineteenth century, from Russia, France, the Netherlands and the United States as well as Britain. On HMS "Beagle", which provided Charles Darwin with ideas and materials for his 1859 book "On the Origin of Species", the ship's captain, Robert FitzRoy, charted the seas and coasts and published his four-volume report of the ship's three voyages in 1839. Edward Forbes's 1854 book, "Distribution of Marine Life" argued that no life could exist below around 600 metres (2000 feet). This was proven wrong by the British biologists W. B. Carpenter and C. Wyville Thomson, who in 1868 discovered life in deep water by dredging. Wyville Thompson became chief scientist on the Challenger expedition of 1872–1876, which effectively created the science of oceanography.


title: Sea

text: On her journey round the globe, "HMS Challenger" discovered about 4,700 new marine species, and made 492 deep sea soundings, 133 bottom dredges, 151 open water trawls and 263 serial water temperature observations. In the southern Atlantic in 1898/1899, Carl Chun on the "Valdivia" brought many new life forms to the surface from depths of over . The first observations of deep-sea animals in their natural environment were made in 1930 by William Beebe and Otis Barton who descended to in the spherical steel Bathysphere. This was lowered by cable but by 1960 a self-powered submersible, Trieste developed by Jacques Piccard, took Piccard and Don Walsh to the deepest part of the Earth's oceans, the Mariana Trench in the Pacific, reaching a record depth of about , a feat not repeated until 2012 when James Cameron piloted the Deepsea Challenger to similar depths. An atmospheric diving suit can be worn for deep sea operations, with a new world record being set in 2006 when a US Navy diver descended to in one of these articulated, pressurized suits.


title: Sea

text: At great depths, no light penetrates through the water layers from above and the pressure is extreme. For deep sea exploration it is necessary to use specialist vehicles, either remotely operated underwater vehicles with lights and cameras or crewed submersibles. The battery-operated Mir submersibles have a three-person crew and can descend to 20,000 feet (6,000 m). They have viewing ports, 5,000-watt lights, video equipment and manipulator arms for collecting samples, placing probes or pushing the vehicle across the sea bed when the thrusters would stir up excessive sediment.


title: Sea

text: Bathymetry is the mapping and study of the topography of the ocean floor. Methods used for measuring the depth of the sea include single or multibeam echosounders, laser airborne depth sounders and the calculation of depths from satellite remote sensing data. This information is used for determining the routes of undersea cables and pipelines, for choosing suitable locations for siting oil rigs and offshore wind turbines and for identifying possible new fisheries.


title: Sea

text: Ongoing oceanographic research includes marine lifeforms, conservation, the marine environment, the chemistry of the ocean, the studying and modelling of climate dynamics, the air-sea boundary, weather patterns, ocean resources, renewable energy, waves and currents, and the design and development of new tools and technologies for investigating the deep. Whereas in the 1960s and 1970s research could focus on taxonomy and basic biology, in the 2010s attention has shifted to larger topics such as climate change. Researchers make use of satellite-based remote sensing for surface waters, with research ships, moored observatories and autonomous underwater vehicles to study and monitor all parts of the sea.


title: Sea

text: "Freedom of the seas" is a principle in international law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in international waters. Today, this concept is enshrined in the United Nations Convention on the Law of the Sea (UNCLOS), the third version of which came into force in 1994. Article 87(1) states: "The high seas are open to all states, whether coastal or land-locked." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying of submarine cables, building artificial islands, fishing and scientific research. The safety of shipping is regulated by the International Maritime Organization. Its objectives include developing and maintaining a regulatory framework for shipping, maritime safety, environmental concerns, legal matters, technical co-operation and maritime security.


title: Sea

text: UNCLOS defines various areas of water. "Internal waters" are on the landward side of a baseline and foreign vessels have no right of passage in these. "Territorial waters" extend to 12 nautical miles (22 kilometres; 14 miles) from the coastline and in these waters, the coastal state is free to set laws, regulate use and exploit any resource. A "contiguous zone" extending a further 12 nautical miles allows for hot pursuit of vessels suspected of infringing laws in four specific areas: customs, taxation, immigration and pollution. An "exclusive economic zone" extends for 200 nautical miles (370 kilometres; 230 miles) from the baseline. Within this area, the coastal nation has sole exploitation rights over all natural resources. The "continental shelf" is the natural prolongation of the land territory to the continental margin's outer edge, or 200 nautical miles from the coastal state's baseline, whichever is greater. Here the coastal nation has the exclusive right to harvest minerals and also living resources "attached" to the seabed.


title: Sea

text: Control of the sea is important to the security of a maritime nation, and the naval blockade of a port can be used to cut off food and supplies in time of war. Battles have been fought on the sea for more than 3,000 years. In about 1210 B.C., Suppiluliuma II, the king of the Hittites, defeated and burned a fleet from Alashiya (modern Cyprus). In the decisive 480 B.C. Battle of Salamis, the Greek general Themistocles trapped the far larger fleet of the Persian king Xerxes in a narrow channel and attacked vigorously, destroying 200 Persian ships for the loss of 40 Greek vessels. At the end of the Age of Sail, the British Royal Navy, led by Horatio Nelson, broke the power of the combined French and Spanish fleets at the 1805 Battle of Trafalgar.


title: Sea

text: With steam and the industrial production of steel plate came greatly increased firepower in the shape of the dreadnought battleships armed with long-range guns. In 1905, the Japanese fleet decisively defeated the Russian fleet, which had travelled over , at the Battle of Tsushima. Dreadnoughts fought inconclusively in the First World War at the 1916 Battle of Jutland between the Royal Navy's Grand Fleet and the Imperial German Navy's High Seas Fleet. In the Second World War, the British victory at the 1940 Battle of Taranto showed that naval air power was sufficient to overcome the largest warships, foreshadowing the decisive sea-battles of the Pacific War including the Battles of the Coral Sea, Midway, the Philippine Sea, and the climactic Battle of Leyte Gulf, in all of which the dominant ships were aircraft carriers.


title: Sea

text: Submarines became important in naval warfare in World War I, when German submarines, known as U-boats, sank nearly 5,000 Allied merchant ships, including the RMS Lusitania, which helped to bring the United States into the war. In World War II, almost 3,000 Allied ships were sunk by U-boats attempting to block the flow of supplies to Britain, but the Allies broke the blockade in the Battle of the Atlantic, which lasted the whole length of the war, sinking 783 U-boats. Since 1960, several nations have maintained fleets of nuclear-powered ballistic missile submarines, vessels equipped to launch ballistic missiles with nuclear warheads from under the sea. Some of these are kept permanently on patrol.


title: Sea

text: Sailing ships or packets carried mail overseas, one of the earliest being the Dutch service to Batavia in the 1670s. These added passenger accommodation, but in cramped conditions. Later, scheduled services were offered but the time journeys took depended much on the weather. When steamships replaced sailing vessels, ocean-going liners took over the task of carrying people. By the beginning of the twentieth century, crossing the Atlantic took about five days and shipping companies competed to own the largest and fastest vessels. The Blue Riband was an unofficial accolade given to the fastest liner crossing the Atlantic in regular service. The "Mauretania" held the title with 26.06 knots (48.26 km/h) for twenty years from 1909. The Hales Trophy, another award for the fastest commercial crossing of the Atlantic, was won by the "United States" in 1952 for a crossing that took three days, ten hours and forty minutes.


title: Sea

text: The great liners were comfortable but expensive in fuel and staff. The age of the trans-Atlantic liners waned as cheap intercontinental flights became available. In 1958, a regular scheduled air service between New York and Paris taking seven hours doomed the Atlantic ferry service to oblivion. One by one the vessels were laid up, some were scrapped, others became cruise ships for the leisure industry and still others floating hotels.


title: Sea

text: Maritime trade has existed for millennia. The Ptolemaic dynasty had developed trade with India using the Red Sea ports and in the first millennium BC the Arabs, Phoenicians, Israelites and Indians traded in luxury goods such as spices, gold, and precious stones. The Phoenicians were noted sea traders and under the Greeks and Romans, commerce continued to thrive. With the collapse of the Roman Empire, European trade dwindled but it continued to flourish among the kingdoms of Africa, the Middle East, India, China and southeastern Asia. From the 16th to the 19th centuries, over a period of 400 years, about 12–13 million Africans were shipped across the Atlantic to be sold as slaves in the Americas as part of the Atlantic slave trade.


title: Sea

text: Large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through the Pillars of Hercules, across the Mediterranean and the Suez Canal to the Indian Ocean and through the Straits of Malacca; much trade also passes through the English Channel. Shipping lanes are the routes on the open sea used by cargo vessels, traditionally making use of trade winds and currents. Over 60 percent of the world's container traffic is conveyed on the top twenty trade routes. Increased melting of Arctic ice since 2007 enables ships to travel the Northwest Passage for some weeks in summertime, avoiding the longer routes via the Suez Canal or the Panama Canal.


title: Sea

text: Shipping is supplemented by air freight, a more expensive process mostly used for particularly valuable or perishable cargoes. Seaborne trade carries more than US$4 trillion worth of goods each year. Bulk cargo in the form of liquids, powder or particles are carried loose in the holds of bulk carriers and include crude oil, grain, coal, ore, scrap metal, sand and gravel. Other cargo, such as manufactured goods, is usually transported within standard sized, lockable containers, loaded on purpose-built container ships at dedicated terminals. Before the rise of containerization in the 1960s, these goods were loaded, transported and unloaded piecemeal as break-bulk cargo. Containerization greatly increased the efficiency and decreased the cost of moving goods by sea, and was a major factor leading to the rise of globalization and exponential increases in international trade in the mid-to-late 20th century.


title: Sea

text: Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients. In 2009, 16.6% of the world's intake of animal protein and 6.5% of all protein consumed came from fish. In order to fulfill this need, coastal countries have exploited marine resources in their exclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters. In 2011, the total world production of fish, including aquaculture, was estimated to be 154 million tonnes, of which most was for human consumption. The harvesting of wild fish accounted for 90.4 million tonnes, while annually increasing aquaculture contributes the rest. The north west Pacific is by far the most productive area with 20.9 million tonnes (27 percent of the global marine catch) in 2010. In addition, the number of fishing vessels in 2010 reached 4.36 million, whereas the number of people employed in the primary sector of fish production in the same year amounted to 54.8 million.


title: Sea

text: Modern fishing vessels include fishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels and large factory ships which are designed to stay at sea for weeks, processing and freezing great quantities of fish. The equipment used to capture the fish may be purse seines, other seines, trawls, dredges, gillnets and long-lines and the fish species most frequently targeted are herring, cod, anchovy, tuna, flounder, mullet, squid and salmon. Overexploitation has become a serious concern; it does not only cause the depletion of fish stocks, but also substantially reduce the size of predatory fish populations. It has been estimated that "industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation." In order to avoid overexploitation, many countries have introduced quotas in their own waters. However, recovery efforts often entail substantial costs to local economies or food provision.


title: Sea

text: Artisan fishing methods include rod and line, harpoons, skin diving, traps, throw nets and drag nets. Traditional fishing boats are powered by paddle, wind or outboard motors and operate in near-shore waters. The Food and Agriculture Organization is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.


title: Sea

text: About 79 million tonnes (78M long tons; 87M short tons) of food and non-food products were produced by aquaculture in 2010, an all-time high. About six hundred species of plants and animals were cultured, some for use in seeding wild populations. The animals raised included finfish, aquatic reptiles, crustaceans, molluscs, sea cucumbers, sea urchins, sea squirts and jellyfish. Integrated mariculture has the advantage that there is a readily available supply of planktonic food in the ocean, and waste is removed naturally. Various methods are employed. Mesh enclosures for finfish can be suspended in the open seas, cages can be used in more sheltered waters or ponds can be refreshed with water at each high tide. Shrimps can be reared in shallow ponds connected to the open sea. Ropes can be hung in water to grow algae, oysters and mussels. Oysters can be reared on trays or in mesh tubes. Sea cucumbers can be ranched on the seabed. Captive breeding programmes have raised lobster larvae for release of juveniles into the wild resulting in an increased lobster harvest in Maine. At least 145 species of seaweed – red, green, and brown algae – are eaten worldwide, and some have long been farmed in Japan and other Asian countries; there is great potential for additional algaculture. Few maritime flowering plants are widely used for food but one example is marsh samphire which is eaten both raw and cooked. A major difficulty for aquaculture is the tendency towards monoculture and the associated risk of widespread disease. Aquaculture is also associated with environmental risks; for instance, shrimp farming has caused the destruction of important mangrove forests throughout southeast Asia.


title: Sea

text: Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century. Maritime leisure activities are varied, and include self-organized trips cruising, yachting, powerboat racing and fishing; commercially organized voyages on cruise ships; and trips on smaller vessels for ecotourism such as whale watching and coastal birdwatching.


title: Sea

text: Sea bathing became the vogue in Europe in the 18th century after Dr. William Buchan advocated the practice for health reasons. Surfing is a sport in which a wave is ridden by a surfer, with or without a surfboard. Other marine water sports include kite surfing, where a power kite propels a rider on a board across the water, windsurfing, where the power is provided by a fixed, manoeuvrable sail and water skiing, where a powerboat is used to pull a skier.


title: Sea

text: Beneath the surface, freediving is necessarily restricted to shallow descents. Pearl divers can dive to with baskets to collect oysters. Human eyes are not adapted for use underwater but vision can be improved by wearing a diving mask. Other useful equipment includes fins and snorkels, and scuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface. The depths that can be reached by divers and the length of time they can stay underwater is limited by the increase of pressure they experience as they descend and the need to prevent decompression sickness as they return to the surface. Recreational divers restrict themselves to depths of beyond which the danger of nitrogen narcosis increases. Deeper dives can be made with specialised equipment and training.


title: Sea

text: The sea offers a very large supply of energy carried by ocean waves, tides, salinity differences, and ocean temperature differences which can be harnessed to generate electricity. Forms of sustainable marine energy include tidal power, ocean thermal energy and wave power. Electricity power stations are often located on the coast or beside an estuary so that the sea can be used as a heat sink. A colder heat sink enables more efficient power generation, which is important for expensive nuclear power plants in particular.


title: Sea

text: Tidal power uses generators to produce electricity from tidal flows, sometimes by using a dam to store and then release seawater. The Rance barrage, long, near St Malo in Brittany opened in 1967; it generates about 0.5 GW, but it has been followed by few similar schemes.


title: Sea

text: The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop. A small 2 MW commercial wave power plant, "Osprey", was built in Northern Scotland in 1995 about 300 metres (1000 ft) offshore. It was soon damaged by waves, then destroyed by a storm.


title: Sea

text: Offshore wind power is captured by wind turbines placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore. The first offshore wind farm was installed in Denmark in 1991, and the installed capacity of worldwide offshore wind farms reached 34 GW in 2020, mainly situated in Europe.


title: Sea

text: The seabed contains large reserves of minerals which can be exploited by dredging. This has advantages over land-based mining in that equipment can be built at specialised shipyards and infrastructure costs are lower. Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps. There is a risk of coastal erosion and environmental damage.


title: Sea

text: Seafloor massive sulphide deposits are potential sources of silver, gold, copper, lead and zinc and trace metals since their discovery in the 1960s. They form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract.


title: Sea

text: There are large deposits of petroleum, as oil and natural gas, in rocks beneath the seabed. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land. Offshore oil and gas production can be difficult due to the remote, harsh environment. Drilling for oil in the sea has environmental impacts. Animals may be disorientated by seismic waves used to locate deposits, and there is debate as to whether this causes the beaching of whales. Toxic substances such as mercury, lead and arsenic may be released. The infrastructure may cause damage, and oil may be spilt.


title: Sea

text: Large quantities of methane clathrate exist on the seabed and in ocean sediment, of interest as a potential energy source. Also on the seabed are manganese nodules formed of layers of iron, manganese and other hydroxides around a core. In the Pacific these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly. Their commercial extraction for nickel was investigated in the 1970s but abandoned in favour of more convenient sources. In suitable locations, diamonds are gathered from the seafloor using suction hoses to bring gravel ashore. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above. In Namibia, more diamonds are now collected from marine sources than by conventional methods on land.


title: Sea

text: The sea holds large quantities of valuable dissolved minerals. The most important, Salt for table and industrial use has been harvested by solar evaporation from shallow ponds since prehistoric times. Bromine, accumulated after being leached from the land, is economically recovered from the Dead Sea, where it occurs at 55,000 parts per million (ppm).


title: Sea

text: Desalination is the technique of removing salts from seawater to leave fresh water suitable for drinking or irrigation. The two main processing methods, vacuum distillation and reverse osmosis, use large quantities of energy. Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations. The brine produced as a by-product contains some toxic materials and is returned to the sea.


title: Sea

text: Several nomadic indigenous groups in Maritime Southeast Asia live in boats and derive nearly all they need from the sea. The Moken people live on the coasts of Thailand and Burma and islands in the Andaman Sea. The Bajau people are originally from the Sulu Archipelago, Mindanao and northern Borneo. Some Sea Gypsies are accomplished free-divers, able to descend to depths of , though many are adopting a more settled, land-based way of life.


title: Sea

text: The indigenous peoples of the Arctic such as the Chukchi, Inuit, Inuvialuit and Yup'iit hunt marine mammals including seals and whales, and the Torres Strait Islanders of Australia include the Great Barrier Reef among their possessions. They live a traditional life on the islands involving hunting, fishing, gardening and trading with neighbouring peoples in Papua and mainland Aboriginal Australians.


title: Sea

text: The sea appears in human culture in contradictory ways, as both powerful but serene and as beautiful but dangerous. It has its place in literature, art, poetry, film, theatre, classical music, mythology and dream interpretation. The Ancients personified it, believing it to be under the control of a being who needed to be appeased, and symbolically, it has been perceived as a hostile environment populated by fantastic creatures; the Leviathan of the Bible, Scylla in Greek mythology, Isonade in Japanese mythology, and the kraken of late Norse mythology.


title: Sea

text: The sea and ships have been depicted in art ranging from simple drawings on the walls of huts in Lamu to seascapes by Joseph Turner. In Dutch Golden Age painting, artists such as Jan Porcellis, Hendrick Dubbels, Willem van de Velde the Elder and his son, and Ludolf Bakhuizen celebrated the sea and the Dutch navy at the peak of its military prowess. The Japanese artist Katsushika Hokusai created colour prints of the moods of the sea, including "The Great Wave off Kanagawa".


title: Sea

text: Music too has been inspired by the ocean, sometimes by composers who lived or worked near the shore and saw its many different aspects. Sea shanties, songs that were chanted by mariners to help them perform arduous tasks, have been woven into compositions and impressions in music have been created of calm waters, crashing waves and storms at sea.


title: Sea

text: As a symbol, the sea has for centuries played a role in literature, poetry and dreams. Sometimes it is there just as a gentle background but often it introduces such themes as storm, shipwreck, battle, hardship, disaster, the dashing of hopes and death. In his epic poem the "Odyssey", written in the eighth century BC, Homer describes the ten-year voyage of the Greek hero Odysseus who struggles to return home across the sea's many hazards after the war described in the "Iliad". The sea is a recurring theme in the Haiku poems of the Japanese Edo period poet Matsuo Bashō (松尾 芭蕉) (1644–1694). In the works of psychiatrist Carl Jung, the sea symbolizes the personal and the collective unconscious in dream interpretation, the depths of the sea symbolizing the depths of the unconscious mind.


title: Sea

text: Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.


title: Sea

text: Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300 million years. More recently, climate change has resulted in an increase of the carbon dioxide content of the atmosphere; about 30–40% of the added CO is absorbed by the oceans, forming carbonic acid and lowering the pH (now below 8.1) through a process called ocean acidification. The pH is expected to reach 7.7 (representing a 3-fold increase in hydrogen ion concentration) by the year 2100, which is a significant change in a century.


title: Sea

text: One important element for the formation of skeletal material in marine animals is calcium, but calcium carbonate becomes more soluble with pressure, so carbonate shells and skeletons dissolve below its compensation depth. Calcium carbonate also becomes more soluble at lower pH, so ocean acidification is likely to have profound effects on marine organisms with calcareous shells, such as oysters, clams, sea urchins, and corals, because their ability to form shells will be reduced, and the carbonate compensation depth will rise closer to the sea surface. Affected planktonic organisms will include the snail-like molluscs known as pteropods, and single-celled algae called coccolithophorids and foraminifera. All of these are important parts of the food chain and a diminution in their numbers will have significant consequences. In tropical regions, corals are likely to be severely affected as it becomes more difficult to build their calcium carbonate skeletons, in turn adversely impacting other reef dwellers.


title: Sea

text: The current rate of ocean chemistry change appears to be without precedent in Earth's geological history, making it unclear how well marine ecosystems will be able to adapt to the shifting conditions of the near future. Of particular concern is the manner in which the combination of acidification with the expected additional stressors of higher temperatures and lower oxygen levels will impact the seas.


title: Sea

text: Many substances enter the sea as a result of human activities. Combustion products are transported in the air and deposited into the sea by precipitation. Industrial outflows and sewage contribute heavy metals, pesticides, PCBs, disinfectants, household cleaning products and other synthetic chemicals. These become concentrated in the surface film and in marine sediment, especially estuarine mud. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects. The heavy metals of greatest concern are copper, lead, mercury, cadmium and zinc which may be bio-accumulated by marine organisms and are passed up the food chain.


title: Sea

text: Much floating plastic rubbish does not biodegrade, instead disintegrating over time and eventually breaking down to the molecular level. Rigid plastics may float for years. In the centre of the Pacific gyre there is a permanent floating accumulation of mostly plastic waste and there is a similar garbage patch in the Atlantic. Foraging sea birds such as the albatross and petrel may mistake debris for food, and accumulate indigestible plastic in their digestive systems. Turtles and whales have been found with plastic bags and fishing line in their stomachs. Microplastics may sink, threatening filter feeders on the seabed.


title: Sea

text: Most oil pollution in the sea comes from cities and industry. Oil is dangerous for marine animals. It can clog the feathers of sea birds, reducing their insulating effect and the birds' buoyancy, and be ingested when they preen themselves in an attempt to remove the contaminant. Marine mammals are less seriously affected but may be chilled through the removal of their insulation, blinded, dehydrated or poisoned. Benthic invertebrates are swamped when the oil sinks, fish are poisoned and the food chain is disrupted. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected and the livelihoods of people dependent on the sea being devastated. The marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove oil from the sea. In the Gulf of Mexico, where oil-eating bacteria are already present, they take only a few days to consume spilt oil.


title: Sea

text: Run-off of fertilisers from agricultural land is a major source of pollution in some areas and the discharge of raw sewage has a similar effect. The extra nutrients provided by these sources can cause excessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms and red tides can lower the oxygen level of the water and kill marine animals. Such events have created dead zones in the Baltic Sea and the Gulf of Mexico. Some algal blooms are caused by cyanobacteria that make shellfish that filter feed on them toxic, harming animals like sea otters. Nuclear facilities too can pollute. The Irish Sea was contaminated by radioactive caesium-137 from the former Sellafield nuclear fuel processing plant and nuclear accidents may also cause radioactive material to seep into the sea, as did the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011.


title: Sea

text: The dumping of waste (including oil, noxious liquids, sewage and garbage) at sea is governed by international law. The London Convention (1972) is a United Nations agreement to control ocean dumping which had been ratified by 89 countries by 8 June 2012. MARPOL 73/78 is a convention to minimize pollution of the seas by ships. By May 2013, 152 maritime nations had ratified MARPOL.


title: Copper(II) sulfate

text: Copper(II) sulfate, also known as copper sulphate, is an inorganic compound with the chemical formula . It forms hydrates , where "n" can range from 1 to 7. The pentahydrate ("n" = 5), a bright blue crystal, is the most commonly encountered hydrate of copper(II) sulfate. Older names for the pentahydrate include blue vitriol, bluestone, vitriol of copper, and Roman vitriol. It exothermically dissolves in water to give the aquo complex , which has octahedral molecular geometry. The structure of the solid pentahydrate reveals a polymeric structure wherein copper is again octahedral but bound to four water ligands. The centers are interconnected by sulfate anions to form chains. Anhydrous copper sulfate is a light grey powder.


title: Copper(II) sulfate

text: Copper sulfate is produced industrially by treating copper metal with hot concentrated sulfuric acid or copper oxides with dilute sulfuric acid. For laboratory use, copper sulfate is usually purchased. Copper sulfate can also be produced by slowly leaching low-grade copper ore in air; bacteria may be used to hasten the process.


title: Copper(II) sulfate

text: Commercial copper sulfate is usually about 98% pure copper sulfate, and may contain traces of water. Anhydrous copper sulfate is 39.81 percent copper and 60.19 percent sulfate by mass, and in its blue, hydrous form, it is 25.47% copper, 38.47% sulfate (12.82% sulfur) and 36.06% water by mass. Four types of crystal size are provided based on its usage: large crystals (10–40 mm), small crystals (2–10 mm), snow crystals (less than 2 mm), and windswept powder (less than 0.15 mm).


title: Copper(II) sulfate

text: Copper(II) sulfate pentahydrate decomposes before melting. It loses two water molecules upon heating at , followed by two more at and the final water molecule at .


title: Copper(II) sulfate

text: The chemistry of aqueous copper sulfate is simply that of copper aquo complex, since the sulfate is not bound to copper in such solutions. Thus, such solutions react with concentrated hydrochloric acid to give tetrachlorocuprate(II):


title: Copper(II) sulfate

text: Similarly treatment of such solutions with zinc gives metallic copper, as described by this simplified equation:


title: Copper(II) sulfate

text: A further illustration of such "single metal replacement reactions" occurs when a piece of iron is submerged in a solution of copper sulfate:


title: Copper(II) sulfate

text: In high school and general chemistry education, copper sulfate is used as an electrolyte for galvanic cells, usually as a cathode solution. For example, in a zinc/copper cell, copper ion in copper sulfate solution absorbs electron from zinc and forms metallic copper.


title: Copper(II) sulfate

text: Copper sulfate is commonly included in teenager chemistry sets and undergraduate experiments. It is often used to grow crystals in schools and in copper plating experiments, despite its toxicity. Copper sulfate is often used to demonstrate an exothermic reaction, in which steel wool or magnesium ribbon is placed in an aqueous solution of . It is used to demonstrate the principle of mineral hydration. The pentahydrate form, which is blue, is heated, turning the copper sulfate into the anhydrous form which is white, while the water that was present in the pentahydrate form evaporates. When water is then added to the anhydrous compound, it turns back into the pentahydrate form, regaining its blue color. Copper(II) sulfate pentahydrate can easily be produced by crystallization from solution as copper(II) sulfate, which is hygroscopic.


title: Copper(II) sulfate

text: Copper sulfate has been used for control of algae in lakes and related fresh waters subject to eutrophication. It "remains the most effective algicidal treatment".


title: Copper(II) sulfate

text: Bordeaux mixture, a suspension of copper(II) sulfate () and calcium hydroxide (), is used to control fungus on grapes, melons, and other berries. It is produced by mixing a water solution of copper sulfate and a suspension of slaked lime.


title: Copper(II) sulfate

text: A dilute solution of copper sulfate is used to treat aquarium fishes for parasitic infections, and is also used to remove snails from aquariums and zebra mussels from water pipes. Copper ions are highly toxic to fish, however. Most species of algae can be controlled with very low concentrations of copper sulfate.


title: Copper(II) sulfate

text: Several chemical tests utilize copper sulfate. It is used in Fehling's solution and Benedict's solution to test for reducing sugars, which reduce the soluble blue copper(II) sulfate to insoluble red copper(I) oxide. Copper(II) sulfate is also used in the Biuret reagent to test for proteins.


title: Copper(II) sulfate

text: Copper sulfate is used to test blood for anemia. The blood is dropped into a solution of copper sulfate of known specific gravity—blood with sufficient hemoglobin sinks rapidly due to its density, whereas blood which sinks slowly or not at all has an insufficient amount of hemoglobin.


title: Copper(II) sulfate

text: In a flame test, the copper ions of copper sulfate emit a deep green light, a much deeper green than the flame test for barium.


title: Copper(II) sulfate

text: Copper sulfate is employed at a limited level in organic synthesis. The anhydrous salt is used as a dehydrating agent for forming and manipulating acetal groups. The hydrated salt can be intimately mingled with potassium permanganate to give an oxidant for the conversion of primary alcohols.


title: Copper(II) sulfate

text: Reaction with ammonium hydroxide yields tetraamminecopper(II) sulfate or Schweizer's reagent which was used to dissolve cellulose in the industrial production of Rayon.


title: Copper(II) sulfate

text: Copper(II) sulfate has attracted many niche applications over the centuries. In industry copper sulfate has multiple applications. In printing it is an additive to book-binding pastes and glues to protect paper from insect bites; in building it is used as an additive to concrete to improve water resistance and discourage anything from growing on it. Copper sulfate can be used as a coloring ingredient in artworks, especially glasses and potteries. Copper sulfate is also used in firework manufacture as a blue coloring agent, but it is not safe to mix copper sulfate with chlorates when mixing firework powders.


title: Copper(II) sulfate

text: Copper sulfate was once used to kill bromeliads, which serve as mosquito breeding sites. Copper sulfate is used as a molluscicide to treat bilharzia in tropical countries.


title: Copper(II) sulfate

text: In 2008, the artist Roger Hiorns filled an abandoned waterproofed council flat in London with 75,000 liters of copper(II) sulfate water solution. The solution was left to crystallize for several weeks before the flat was drained, leaving crystal-covered walls, floors and ceilings. The work is titled "Seizure". Since 2011, it has been on exhibition at the Yorkshire Sculpture Park.


title: Copper(II) sulfate

text: Copper(II) sulfate can be used as a mordant in vegetable dyeing. It often highlights the green tints of the specific dyes.


title: Copper(II) sulfate

text: An aqueous solution of copper(II) sulfate is often used as the resistive element in liquid resistors.


title: Copper(II) sulfate

text: Anhydrous copper(II) sulfate can be produced by dehydration of the commonly available pentahydrate copper sulfate. In nature, it is found as the very rare mineral known as chalcocyanite. The pentahydrate also occurs in nature as chalcanthite. Other rare copper sulfate minerals include bonattite (trihydrate), boothite (heptahydrate), and the monohydrate compound poitevinite. There are numerous other, more complex, copper(II) sulfate minerals known, with environmentally important basic copper(II) sulfates like langite and posnjakite.


title: Copper(II) sulfate

text: Copper(II) salts have an LD50 of 100 mg/kg. It is harmless enough to be a routine component of high school experiments and to be used widely in swimming lakes to control algae.


title: Copper(II) sulfate

text: Copper(II) sulfate was used in the past as an emetic. It is now considered too toxic for this use. It is still listed as an antidote in the World Health Organization's Anatomical Therapeutic Chemical Classification System.


title: Diahnne Abbott

text: Diahnne Eugenia Abbott (born January 1, 1945) is an American actress and singer. She played supporting roles in films of the 1970s and 1980s, including "Taxi Driver" (1976).


title: Diahnne Abbott

text: Abbott was married to actor Robert De Niro from 1976 to 1988. They had a son, Raphael, who was named after the hotel in Rome where he was conceived. De Niro adopted Drena, Abbott's daughter from a previous marriage. Drena has appeared in several of her father's films, including "Showtime" (2002), "Wag the Dog" (1997), "City by the Sea" (2002) and "The Intern" (2015). De Niro and Abbott divorced in 1988.


title: Diahnne Abbott

text: Abbott portrayed the pornographic movie theatre box office clerk in Martin Scorsese's "Taxi Driver" (1976) opposite De Niro. She has a memorable cameo in the 1977 film "New York, New York", in which she sings Fats Waller's song, "Honeysuckle Rose".


title: Diahnne Abbott

text: She also played the object of De Niro's affections in Scorsese's 1983 film, "The King of Comedy", as well as roles in the television series "Crime Story" (the character of Sonia) and the 1986 film "Jo Jo Dancer, Your Life Is Calling" (Mother). She is the cousin of singer Gregory Abbott, known for his 1986 song "Shake You Down".


title: Simple Mail Transfer Protocol

text: The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per . For retrieving messages, IMAP (which replaced the older POP3) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.


title: Simple Mail Transfer Protocol

text: SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications).


title: Simple Mail Transfer Protocol

text: Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers. As more computers were interconnected, especially in the U.S. Government's ARPANET, standards were developed to permit exchange of messages between different operating systems. SMTP grew out of these standards developed during the 1970s.


title: Simple Mail Transfer Protocol

text: Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in ; and the SNDMSG program, which Ray Tomlinson of BBN adapted that year to send messages across two computers on the ARPANET. A further proposal for a Mail Protocol was made in RFC 524 in June 1973, which was not implemented.


title: Simple Mail Transfer Protocol

text: The use of the File Transfer Protocol (FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed.


title: Simple Mail Transfer Protocol

text: In 1980, Jon Postel and Suzanne Sluizer published which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP, an allocation that has since been removed by IANA. In November 1981, Postel published "Simple Mail Transfer Protocol".


title: Simple Mail Transfer Protocol

text: The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities.


title: Simple Mail Transfer Protocol

text: SMTP became widely used in the early 1980s. At the time, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both used a store and forward mechanism and are examples of push technology. Though Usenet's newsgroups were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the "bang paths" it used as message routing headers.


title: Simple Mail Transfer Protocol

text: Sendmail, released with 4.1cBSD in 1983, was one of the first mail transfer agents to implement SMTP. Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent).


title: Simple Mail Transfer Protocol

text: The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack, spoofing, and spamming, and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server was an open mail relay. The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002. Because of spam concerns most email providers blocklist open relays, making original SMTP essentially impractical for general use on the Internet.


title: Simple Mail Transfer Protocol

text: In November 1995, defined Extended Simple Mail Transfer Protocol (ESMTP), which established a general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions.


title: Simple Mail Transfer Protocol

text: Message submission () and SMTP-AUTH () were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for the organization "from the outside", and relaying messages from the organization "to the outside". But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become message submission agents for Mail user agents, some of which were now relaying mail "from the outside" of an organization. (e.g. a company executive wishes to send email while on a trip using the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web, meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email (spam). Work on message submission () was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding a domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as a way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices.


title: Simple Mail Transfer Protocol

text: As this protocol started out purely ASCII text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8-bit clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP. Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding is needed for most non-text data and some text formats). In 2012, the codice_1 extension was created to support UTF-8 text, allowing international content and addresses in non-Latin scripts like Cyrillic or Chinese.


title: Simple Mail Transfer Protocol

text: Many people contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore.


title: Simple Mail Transfer Protocol

text: Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host. Each process is an MTA (an SMTP server) in its own right.


title: Simple Mail Transfer Protocol

text: The boundary MTA uses DNS to look up the MX (mail exchanger) record for the recipient's domain (the part of the email address on the right of codice_2). The MX record contains the name of the target MTA. Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange.


title: Simple Mail Transfer Protocol

text: Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be the ultimate destination, an intermediate "relay" (that is, it stores and forwards the message) or a "gateway" (that is, it may forward the message using some protocol other than SMTP). Per section 2.1, each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so.


title: Simple Mail Transfer Protocol

text: Once the final hop accepts the incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP), a derivative of SMTP designed for this purpose.


title: Simple Mail Transfer Protocol

text: Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses the traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes/Domino. Webmail clients may use either method, but the retrieval protocol is often not a formal standard.


title: Simple Mail Transfer Protocol

text: SMTP defines message "transport", not the message "content". Thus, it defines the mail "envelope" and its parameters, such as the envelope sender, but not the header (except "trace information") nor the body of the message itself. STD 10 and define SMTP (the envelope), while STD 11 and define the message (header and body), formally referred to as the Internet Message Format.


title: Simple Mail Transfer Protocol

text: SMTP is a connection-oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection. An "SMTP session" consists of commands originated by an SMTP client (the initiating agent, sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An "SMTP transaction" consists of three command/reply sequences:


title: Simple Mail Transfer Protocol

text: Besides the intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A reject is a permanent failure and the client should send a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than delivery.


title: Simple Mail Transfer Protocol

text: The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.


title: Simple Mail Transfer Protocol

text: A MUA knows the "outgoing mail" SMTP server from its configuration. A relay server typically determines which server to connect to by looking up the MX (Mail eXchange) DNS resource record for each recipient's domain name. If no MX record is found, a conformant relaying server (not all are) instead looks up the A record. Relay servers can also be configured to use a smart host. A relay server initiates a TCP connection to the server on the "well-known port" for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication.


title: Simple Mail Transfer Protocol

text: SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users retrieving messages and managing mail boxes. To permit an intermittently-connected mail server to "pull" messages from a remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see Remote Message Queue Starting below). POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed.


title: Simple Mail Transfer Protocol

text: Remote Message Queue Starting enables a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command. The original codice_3 command was deemed insecure and was extended in with the codice_4 command which operates more securely using an authentication method based on Domain Name System information.


title: Simple Mail Transfer Protocol

text: An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as a DNS name). This server will deliver outgoing messages on behalf of the user.


title: Simple Mail Transfer Protocol

text: Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example spam. Two solutions have been in common use:


title: Simple Mail Transfer Protocol

text: Under this system, an ISP's SMTP server will not allow access by users who are outside the ISP's network. More precisely, the server may only allow access to users with an IP address provided by the ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP. A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible.


title: Simple Mail Transfer Protocol

text: This system has several variations. For example, an organisation's SMTP server may only provide service to users on the same network, enforcing this by firewalling to block access by users on the wider Internet. Or the server may perform range checks on the client's IP address. These methods were typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation. However, most of these bodies now use client authentication methods, as described below.


title: Simple Mail Transfer Protocol

text: Where a user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical. It is highly desirable to be able to use email client configuration information that does not need to change.


title: Simple Mail Transfer Protocol

text: Modern SMTP servers typically require authentication of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server. SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication mechanism.


title: Simple Mail Transfer Protocol

text: Mail "clients" however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of:


title: Simple Mail Transfer Protocol

text: Port 2525 and others may be used by some individual providers, but have never been officially supported.


title: Simple Mail Transfer Protocol

text: Many Internet service providers now block all outgoing port 25 traffic from their customers. Mainly as an anti-spam measure, but also to cure for the higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open.


title: Simple Mail Transfer Protocol

text: A typical example of sending a message via SMTP to two mailboxes ("alice" and "theboss") located in the same mail domain ("example.com") is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with "S:" and "C:", for "server" and "client", respectively; these labels are not part of the exchange.)


title: Simple Mail Transfer Protocol

text: After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its fully qualified domain name (FQDN), in this case "smtp.example.com". The client initiates its dialog by responding with a codice_5 command identifying itself in the command's parameter with its FQDN (or an address literal if none is available).


title: Simple Mail Transfer Protocol

text: The client notifies the receiver of the originating email address of the message in a codice_6 command. This is also the return or bounce address in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the codice_7 and codice_8 header fields. The corresponding SMTP command is codice_9. Each successful reception and execution of a command is acknowledged by the server with a result code and response message (e.g., codice_10).


title: Simple Mail Transfer Protocol

text: The transmission of the body of the mail message is initiated with a codice_11 command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This sequence consists of a new-line (codice_12), a single full stop (codice_13), followed by another new-line (codice_12). Since a message body can contain a line with just a period as part of the text, the client sends "two" periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called "dot-stuffing".


title: Simple Mail Transfer Protocol

text: The server's positive reply to the end-of-data, as exemplified, implies that the server has taken the responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power shortage: Until the sender has received that codice_10 reply, it must assume the message was not delivered. On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of the message that they will try to deliver. The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.


title: Simple Mail Transfer Protocol

text: The codice_16 command ends the session. If the email has other recipients located elsewhere, the client would codice_16 and connect to an appropriate SMTP server for subsequent recipients after the current destination(s) had been queued. The information that the client sends in the codice_5 and codice_6 commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a codice_20 and codice_21 header field, respectively.


title: Simple Mail Transfer Protocol

text: Some clients are implemented to close the connection after the message is accepted (codice_22), so the last two lines may actually be omitted. This causes an error on the server when trying to send the codice_23 reply.


title: Simple Mail Transfer Protocol

text: Clients learn a server's supported options by using the codice_24 greeting, as exemplified below, instead of the original codice_5. Clients fall back to codice_5 only if the server does not support codice_24 greeting.


title: Simple Mail Transfer Protocol

text: Modern clients may use the ESMTP extension keyword codice_28 to query the server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute.


title: Simple Mail Transfer Protocol

text: Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the codice_5 command with the codice_24 command.


title: Simple Mail Transfer Protocol

text: Thus "smtp2.example.com" declares that it can accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes).


title: Simple Mail Transfer Protocol

text: In the simplest case, an ESMTP server declares a maximum codice_28 immediately after receiving an codice_24. According to , however, the numeric parameter to the codice_28 extension in the codice_24 response is optional. Clients may instead, when issuing a codice_6 command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages.


title: Simple Mail Transfer Protocol

text: Original SMTP supports only a single body of ASCII text, therefore any binary data needs to be encoded as text into that body of the message before transfer, and then decoded by the recipient. Binary-to-text encodings, such as uuencode and BinHex were typically used.


title: Simple Mail Transfer Protocol

text: The 8BITMIME command was developed to address this. It was standardized in 1994 as It facilitates the transparent exchange of e-mail messages containing octets outside the seven-bit ASCII character set by encoding them as MIME content parts, typically encoded with Base64.


title: Simple Mail Transfer Protocol

text: On-Demand Mail Relay (ODMR) is an SMTP extension standardized in that allows an intermittently-connected SMTP server to receive email queued for it when it is connected.


title: Simple Mail Transfer Protocol

text: Original SMTP supports email addresses composed of ASCII characters only, which is inconvenient for users whose native script is not Latin based, or who use diacritic not in the ASCII character set. This limitation was alleviated via extensions enabling UTF-8 in address names. introduced experimental codice_36 command and later was superseded by that introduced codice_1 command. These extensions provide support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as Greek and Chinese.


title: Simple Mail Transfer Protocol

text: Current support is limited, but there is strong interest in broad adoption of and the related RFCs in countries like China that have a large user base where Latin (ASCII) is a foreign script.


title: Simple Mail Transfer Protocol

text: Like SMTP, ESMTP is a protocol used to transport Internet mail. It is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol.


title: Simple Mail Transfer Protocol

text: The main identification feature for ESMTP clients is to open a transmission with the command codice_24 (Extended HELLO), rather than codice_5 (Hello, the original standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821 compliant server returns error code 500, allowing ESMTP clients to try either codice_5 or codice_16.


title: Simple Mail Transfer Protocol

text: Each service extension is defined in an approved format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA). The first definitions were the RFC 821 optional services: codice_42, codice_43 (Send or Mail), codice_44 (Send and Mail), codice_45, codice_46, and codice_3. The format of additional SMTP verbs was set and for new parameters in codice_48 and codice_49.


title: Simple Mail Transfer Protocol

text: The ESMTP format was restated in (superseding RFC 821) and updated to the latest definition in in 2008. Support for the codice_24 command in servers became mandatory, and codice_5 designated a required fallback.


title: Simple Mail Transfer Protocol

text: Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an codice_24 message keyword starting with "X", and with any additional parameters or verbs similarly marked.


title: Simple Mail Transfer Protocol

text: SMTP commands are case-insensitive. They are presented here in capitalized form for emphasis only. An SMTP server that requires a specific capitalization method is a violation of the standard.


title: Simple Mail Transfer Protocol

text: The following servers can be configured to advertise 8BITMIME, but do not perform conversion of 8-bit data to 7-bit when connecting to non-8BITMIME relays:


title: Simple Mail Transfer Protocol

text: The SMTP-AUTH extension provides an access control mechanism. It consists of an authentication step through which the client effectively logs into the mail server during the process of sending mail. Servers that support SMTP-AUTH can usually be configured to require clients to use this extension, ensuring the true identity of the sender is known. The SMTP-AUTH extension is defined in .


title: Simple Mail Transfer Protocol

text: SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as spammers. It does not necessarily guarantee the authenticity of either the SMTP envelope sender or the "From:" header. For example, spoofing, in which one sender masquerades as someone else, is still possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for.


title: Simple Mail Transfer Protocol

text: The SMTP-AUTH extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail. In general this requires the recipient server to trust the sending server, meaning that this aspect of SMTP-AUTH is rarely used on the Internet.


title: Simple Mail Transfer Protocol

text: Mail delivery can occur both over plain text and encrypted connections, however the communicating parties might not know in advance of other party's ability to use secure channel.


title: Simple Mail Transfer Protocol

text: The STARTTLS extensions enables supporting SMTP servers to notify connecting clients that it supports TLS encrypted communication and offers the opportunity for clients to upgrade their connection by sending the STARTTLS command. Servers supporting the extension do not inherently gain any security benefits from its implementation on its own, as upgrading to a TLS encrypted session is dependent on the connecting client deciding to exercise this option, hence the term "opportunistic" TLS.


title: Simple Mail Transfer Protocol

text: STARTTLS is effective only against passive observation attacks, since the STARTTLS negotiation happens in plain text and an active attacker can trivially remove STARTTLS commands. This type of man-in-the-middle attack is sometimes referred to as STRIPTLS, where the encryption negotiation information sent from one end never reaches the other. In this scenario both parties take the invalid or unexpected responses as indication that the other does not properly support STARTTLS, defaulting to traditional plain-text mail transfer. Note that STARTTLS is also defined for IMAP and POP3 in other RFCs, but these protocols serve different purposes: SMTP is used for communication between message transfer agents, while IMAP and POP3 are for end clients and message transfer agents.


title: Simple Mail Transfer Protocol

text: In 2014 the Electronic Frontier Foundation began "STARTTLS Everywhere" project that, similarly to "HTTPS Everywhere" list, allowed relying parties to discover others supporting secure communication without prior communication. The project stopped accepting submissions on 29 April 2021, and EFF recommended switching to DANE and MTA-STS for discovering information on peers' TLS support.


title: Simple Mail Transfer Protocol

text: A newer 2018 called "SMTP MTA Strict Transport Security (MTA-STS)" aims to address the problem of active adversary by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific DNS TXT records. The relying party would regularly check existence of such record, and cache it for the amount of time specified in the record and never communicate over insecure channels until record expires. Note that MTA-STS records apply only to SMTP traffic between mail servers while communications between a user's client and the mail server are protected by Transport Layer Security with SMTP/MSA, IMAP, POP3, or HTTPS in combination with an organizational or technical policy. Essentially, MTA-STS is a means to extend such a policy to third parties.


title: Simple Mail Transfer Protocol

text: Protocols designed to securely deliver messages can fail due to misconfigurations or deliberate active interference, leading to undelivered messages or delivery over unencrypted or unauthenticated channels. "SMTP TLS Reporting" describes a reporting mechanism and format for sharing statistics and specific information about potential failures with recipient domains. Recipient domains can then use this information to both detect potential attacks and diagnose unintentional misconfigurations.


title: Simple Mail Transfer Protocol

text: The original design of SMTP had no facility to authenticate senders, or check that servers were authorized to send on their behalf, with the result that email spoofing is possible, and commonly used in email spam and phishing.


title: Simple Mail Transfer Protocol

text: Occasional proposals are made to modify SMTP extensively or replace it completely. One example of this is Internet Mail 2000, but neither it, nor any other has made much headway in the face of the network effect of the huge installed base of classic SMTP.


title: Simple Mail Transfer Protocol

text: Instead, mail servers now use a range of techniques, such as stricter enforcement of standards such as , DomainKeys Identified Mail, Sender Policy Framework and DMARC, DNSBLs and greylisting to reject or quarantine suspicious emails.