A view of Earth from above created from images snapped by the Suomi-NPP satellite. 
 
 Even planets can get a bad case of tinnitus, according to a new study that explains Earth's mysterious, never-ending hum. 
 
 Scientists have long known that earthquakes can make the Earth sing like a bell for days or months. However, in the late 1990s, seismologists discovered the world also constantly vibrates at very low frequencies even when there are no quakes. This so-called microseismic activity is too faint for humans to feel. 
 
 Now, researchers say ocean waves are the culprit behind these mysterious tremors. 
 
 Many scientists had previously turned to ocean waves to explain our planet's unusual humming. In one theory, scientists proposed the vibrations were generated by huge ocean waves, which can extend all the way down to the seafloor. The waves can shake the Earth as they tumble over seafloor ridges and underwater continental shelves. Another idea suggested colliding ocean waves triggered the tremors. [In Photos: The 10 Strangest Places on Earth] 
 
 But neither idea could account for the entire range of vibrations seen on earthquake sensors. The new study combines both of these ideas into one model that accounts for these microseismic signals. 
 
 "I think our result is an important step in the transformation of mysterious noise into an understood signal," lead study author Fabrice Ardhuin, an oceanographer at the French Research Institute for Exploitation of the Sea, told Live Science. 
 
 Using computer models of the ocean, winds and seafloor, the scientists found that colliding ocean waves could generate seismic waves that take 13 seconds or less to complete one ripple. When it came to slower waves, they found that ocean waves moving over the seafloor could generate seismic waves with a frequency of 13 to 300 seconds. Most of the mystery hum comes from these longer waves. The pressure of these longer ocean waves on the seafloor causes most of the Earth's bell-like ringing, the researchers said. 
 
 A better understanding of this hum could help scientists generate better maps of the Earth's interior, Ardhuin said. These seismic waves penetrate deep into the planet's mantle and potentially all the way to Earth's core. This means that analyzing these waves could help yield a more detailed picture of the planet's structure, he said. 
 
 The researchers added there could be still more sources of microseismic activity. For instance, perhaps ocean waves traveling along shorelines or traveling down underwater mountains and mid-ocean ridges can generate seismic waves, Ardhuin said. 
 
 Ardhuin and his colleagues Lucia Gualtieri and Eléonore Stutzmann at the Paris Institute of Earth Physics detailed their findings in the Feb. 16 issue of the journal Geophysical Research Letters. 
 
 Follow Live Science @livescience, Facebook & Google+. Original article on Live Science. ||||| Far from the blaring cacophony of cities, towns and suburbs, there are far quieter soundtracks to be found — the murmurs of wind rustling grasses, rushing waves tumbling onto beaches, the creaking of tree branches and trunks. 
 
 But underneath all that is yet another soundscape, a permanent, low-frequency drone produced by Earth itself, from the vibrations of ongoing, subtle seismic movements that are not earthquakes and are too small to be detected without special equipment. 
 
 Earth is "humming." You can't hear it, but it's ongoing. And now scientists have measured that persistent hum from the ocean floor, for the first time. [What's That Noise? 11 Strange and Mysterious Sounds on Earth & Beyond 
 
 Most of the movements in the ground under our feet aren't dramatic enough for people to feel them. Earthquakes, of course, are the big exception, but Earth undergoes far more earthquakes globally than you might suspect — an estimated 500,000 per year, according to the U.S. Geological Survey (USGS). Of those, 100,000 are strong enough to be felt, and about 100 of those are powerful enough to cause damage. 
 
 But even in the quiet periods between earthquakes, there's a whole lot of shaking going on. 
 
 Since the 1990s, researchers have known that Earth is constantly vibrating with microseismic activity, known as "free oscillation," scientists reported in a new study describing new recordings of the phenomenon. Free oscillation creates a hum that can be detected anywhere on land by seismometers — equipment used to detect and record vibrations. 
 
 For years, the source of this perpetual humming stymied researchers, with some suggesting that the rhythmic ebb and flow of ocean waves that reached all the way down to the seafloor were responsible, while others attributed the vibration to collisions between ocean waves. Then, in 2015, scientists determined that both types of ocean movement played a part in keeping Earth vibrating, Live Science previously reported. 
 
 While seismologists have recorded and measured Earth's hum on land, they had yet to capture evidence of the planet's sonic stylings from the ocean depths — until now. 
 
 Recently, scientists traveled to the seafloor in the Indian Ocean to capture the humming sound, using special spherical ocean seismometers. Between September 2012 and November 2013, the researchers deployed 57 free-fall seismometers around La Réunion Island to the east of Madagascar, over an area measuring about 772 square miles (2,000 square kilometers), they wrote in the study. 
 
 Using filters, noise reduction and calculations, they isolated the hum from normal ocean noise levels generated by ocean wave motion and seafloor currents, and found "very clear peaks" that appeared consistently over the 11-month study period, and which appeared in the same amplitude range as measurements of the humming taken on land in Algeria, the scientists reported. They noted that the peaks occurred at several frequencies between 2.9 and 4.5 millihertz — about 10,000 times lower than the human hearing threshold, which is 20 hertz. 
 
 Capturing ocean recordings of Earth's hum will provide scientists with far more data than is currently available from readings taken on land, contributing to efforts to map the planet's interior, the researchers wrote in the study. 
 
 The findings were published online Nov. 27 in the journal Geophysical Research Letters. 
 
 Original article on Live Science. ||||| Scientists have long known that Earth produces an eerie low-frequency hum that's inaudible to humans but detectable with seismic instruments. But as for what's causing this "microseismic" activity, scientists have never been sure. 
 
 Until now. 
 
 A new study published online Feb. 10, 2015 in the journal Geophysical Research Letters indicates that the hum is largely due to ocean waves that cause our planet to vibrate subtly -- or "ring," as the researchers put it. 
 
 "We have made a big step in explaining this," Dr. Fabrice Ardhuin, a senior research scientist at the National Center for Scientific Research in Brest, France and the study's lead author, said in a written statement. "Now we know where this ringing comes from and the next question is: what can we do with it." 
 
 Catching the culprit. With the help of computer models of the ocean, winds, and seafloor, the scientists were able to pinpoint the exact kind of ocean waves that cause the hum and how, Live Science reported. 
 
 The researchers found that collisions between ocean waves generate some seismic activity, but it was mostly the movement and pressure of giant, slow-moving ocean waves that extend all the way down to the seafloor that cause the Earth to hum. 
 
 (Story continues below image.) 
 
 
 
 The graphic on the left shows the computed height of giant waves that can reach the seafloor. The picture on the right shows these types of waves during a storm at a beach south of Bordeaux, France. The pressure of these long ocean waves on the seafloor generates seismic waves that cause the Earth to oscillate, scientists say. 
 
 How the hum can help. The microseismic activity caused by the giant ocean waves penetrates deep into Earth’s mantle -- and possibly to its core. That suggests that by recording it, scientists may be able to gain a more detailed picture of our planet's interior, according to the researchers. 
 
 What would the hum sound like if it were at a higher frequency -- within the range of human hearing? 
 
 "If you played it at 10,000 times the speed you could hear 'white noise' like an old TV set between channels," Ardhuin told The Huffington Post in an email. 
 
 Hmm. Maybe it's a good thing we can't hear the hum.
– Even though we can't feel it, the Earth is humming all the time. "If you played it at 10,000 times the speed, you could hear 'white noise' like an old TV set between channels," Fabrice Ardhuin, an oceanographer in France, tells the Huffington Post. Experts discovered the planet's very low vibrations in the late 1990s, and now researchers may be able to explain them, Live Science reports. According to Ardhuin's team, the hum is a result of ocean waves. Waves don't just occur at the ocean's surface, Live Science explains: They can reach to the bottom of the ocean. There, they move along a "bumpy" sea floor, creating pressure that leads to the Earth oscillating, the American Geophysical Union reports. That could explain the so-called "microseismic" waves behind the hum—at least those lasting from 13 to 300 seconds, according to researchers' models. Shorter microseismic waves could be the result of waves crashing into each other. The wave theory isn't entirely new, Live Science notes, but the findings offer a fuller explanation of what could be happening. "I think (the results are) a relief to the seismologists," says the oceanographer. "Now we know where this ringing comes from, and the next question is: What can we do with it?" One possibility: The research could help experts spot earthquakes occurring a long distance away, AGU notes. (In other recent geological finds, it seems the Earth's core has a core of its own.)