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They discovered features in the data that suggested .?.?. continuous signals,” a University of California at Santa Barbara researcher recounted in 2001. These seismic waves ranged from 2 to 7 millihertz — thousands of times lower than the human hearing range — and continued endlessly, regardless of earthquakes.
[A newly discovered moon tunnel could be the perfect place for a colony, scientists say]
The phenomenon became popularly known as the “hum of the Earth.”
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Webb was one of many researchers who searched for the hum's cause in the 21st century. Some thought interactions between the atmosphere and solid ground caused the shaking, though he discounts the idea.
Rather, Webb said, most recent research suggests the primary cause is ocean waves — “banging on the sea floor pretty much all the way around the Earth.”
Sometimes waves sloshing in opposite directions intersect, sending vibrations deep down into Earth's crust. Sometimes a wave on a shallow coast somewhere ripples over the rough sea floor and adds its own frequencies to the hum.
“I think our result is an important step in the transformation of mysterious noise into an understood signal,” an oceanographer with the French Research Institute for Exploitation of the Sea told Live Science after publishing a 2015 paper detailing the ocean wave theories.
Whatever the origin, the result is a harmony of ultralow frequencies that resonate almost identically all over the globe — and that's potentially invaluable to those who want to know what goes on beneath its surface, where the core spins and tectonic plates shift.
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Scientists already measure how fast earthquake waves travel through different regions of the underground to make detailed subterranean maps.