It's comforting to think of Earth as solid and immovable, but that's false. The world is vibrating, stretching and compressing. We're shaking right along with it.
“The earth is ringing like a bell all the time,” said Spahr Webb, a seismologist at Columbia University.
The hum is everywhere. Its ultralow frequencies have been recorded in Antarctica and Algeria, and — as announced this week by the American Geophysical Union — on the floor of the Indian Ocean. We still don't know what causes it. Some have theorized that it's the echo of colliding ocean waves, or the movements of the atmosphere, or vibrations born of sea and sky alike.
But if we could hear this music more clearly, scientists around the world say, it could reveal deep secrets about the earth beneath us, or even teach us to map out alien planets.
And the hum is getting clearer all the time.
Earth vibrates at different frequencies and amplitudes, for different reasons, and not all those vibrations are the 'hum'. Earthquakes are like huge gong bangs. When an enormous quake hit Japan in 2011, Webb said, the globe kept ringing for a month afterward. People sitting on the other side of the world bounced up and down about a centimeter, though so slowly they didn't feel a thing.
In 1998, a team of researchers analyzed data from a gravimeter in east Antarctica and realized that some of these vibrations never actually stop.
“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.
The phenomenon became popularly known as the “hum of the Earth.”
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.
Scientists already measure how fast earthquake waves travel through different regions of the underground to make detailed subterranean maps.
But earthquakes come randomly and briefly, like flashes of lightning on a dark night. A constant, uniform vibration could act like a floodlight into the underworld.
Some researchers believe the hum extends all the way down to the Earth's core, and some have even fantasized about using hums on other planets to map out alien geography.
And yet we're still only beginning to understand our planet's hum. And scientists have been limited for years because they only knew how to measure it from land, while nearly three-quarters of the globe is underwater.
That's where the new study comes in. A team led by Martha Deen, a geophysicist at the Paris Institute of Earth Physics in Paris, published it last month in the American Geophysical Union's journal.
The scientists collected data from seismometer stations that had been placed in the Indian Ocean near Madagascar several years ago. These stations were meant to study volcanic hot spots — nothing to do with the hum — but the team worked out a method to clean the data of ocean currents, waves, glitches and other noise.
They “were able to reduce the noise level to approximately the same level as a quiet land station,” the Geophysical Union said in an accompanying article.
And when they were done, they were left with the first-ever underwater recording of the hum.
It peaked between 2.9 and 4.5 millihertz, they said — a tighter range than the first hum researchers in the 1990s had recorded. It was also similar to measurements taken from a land-based station in Algeria.
So — more evidence that the hum goes all the way around the world; and more hope that we may one day reveal all that goes on beneath it.
This article has been updated.