Hot spring

A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater that rises from the Earth's crust. While some of these springs contain water that is a safe temperature for bathing, others are hot enough that immersion can be harmful, i.e., lead to scalding and, potentially, death.
Definitions.
There is no universally accepted definition of a hot spring. For example, one can find the phrase "hot spring" defined as 
The related term "warm spring" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al. (2003) suggest that the phrase "warm spring" is not useful and should be avoided. The US NOAA Geophysical Data Center defines a "warm spring" as a spring with water between .
Sources of heat.
Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. 
Water issuing from a hot spring is heated geothermally, that is, with heat produced from the Earth's mantle. In general, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. The water from hot springs in non-volcanic areas is heated in this manner.
In active volcanic zones such as Yellowstone National Park, water may be heated by coming into contact with magma (molten rock). The high temperature gradient near magma may cause water to be heated enough that it boils or becomes superheated. If the water becomes so hot that it builds steam pressure and periodically erupts in a jet above the surface of the Earth, it is called a geyser. If the water only reaches the surface in the form of steam, it is called a fumarole. If the water is mixed with mud and clay, it is called a mud pot.
Note that hot springs in volcanic areas are often at or near the boiling point. People have been seriously scalded and even killed by accidentally or intentionally entering these springs.
Warm springs are sometimes the result of hot and cold springs mixing. They may occur within a volcanic area or outside of one. One example of a non-volcanic warm spring is Warm Springs, Georgia (frequented for its therapeutic effects by paraplegic U.S. President Franklin D. Roosevelt, who built the Little White House there).
Flow rates.
Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a geyser, or fountain.
High-flow hot springs.
There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Springs with high flow rates include:
Therapeutic uses.
Because heated water can hold more dissolved solids than cold water, warm and especially hot springs, including beneficial sulphurous water, often with very high mineral content, containing everything from simple calcium to lithium, and even radium. Because of both the folklore and the claimed medical value some of these springs have, they are often popular tourist destinations, and locations for rehabilitation clinics for those with disabilities.
Precautions.
A thermophile is an organism — a type of extremophile — that thrives at high temperatures, between . Thermophiles are found in hot springs, as well as deep sea hydrothermal vents and decaying plant matter such as peat bogs and compost.
Some hot springs microbiota are infectious to humans:
Examples.
There are hot springs in many places and on all continents of the world. Countries that are renowned for their hot springs include China, Costa Rica, Iceland, Iran, Japan, New Zealand, Brazil, Peru, Taiwan, Turkey, and the United States, but there are hot springs in many other places as well:
Etiquette.
The customs and practices observed differ depending on the hot spring. It is common practice that bathers should wash before entering the water so as not to contaminate the water (with/without soap). In many countries, like Japan, it is required to enter the hot spring with no clothes on, including swimwear. Typically in these circumstances, there are different facilities or times for men and women. In some countries, if it is a public hot spring, swimwear is required.