Earth gives off a relentless hum of countless notes completely imperceptible to the human ear, like a giant, exceptionally quiet symphony, but the origin of this sound remains a mystery.

Now unexpected powerful tunes have been discovered in this hum. These new findings could shed light on the source of this enigma.

The planet emanates a constant rumble far below the limits of human hearing, even when the ground isn't shaking from an earthquake. (It does not cause the ringing in the ear linked with tinnitus.) This sound, first discovered a decade ago, is one that only scientific instruments — seismometers — can detect. Researchers call it Earth's hum.

Investigators suspect this murmur could originate from the churning ocean, or perhaps the roiling atmosphere. To find out more, scientists analyzed readings from an exceptionally quiet Earth-listening research station at the Black Forest Observatory in Germany, with supporting data from Japan and China.

{{ video="080415-earth-hum" title="Video - Earth's Hum" caption="Our planet makes ring-shaped oscillations that act like a very quiet symphony. Credit: Credit: Rudolf Widmer-Schnidrig" }}

Different types

In the past, the oscillations that researchers found made up this hum were "spheroidal" — they basically involved patches of rock moving up and down, albeit near undetectably.

Now oscillations have been discovered making up the hum that, oddly, are shaped roughly like rings. Imagine, if you will, rumbles that twist in circles in rock across the upper echelons of the planet, almost like dozens of lazy hurricanes.

Scientists had actually expected to find these kinds of oscillations, but these new ring-like waves are surprisingly about as powerful as the spheroidal ones are. The expectation was they would be relatively insignificant.

New thinking

This discovery should force researchers to significantly rethink what causes Earth's hum. While the spheroidal oscillations might be caused by forces squeezing down on the planet — say, pressure from ocean or atmospheric waves — the twisting ring-like phenomena might be caused by forces shearing across the world's surface, from the oceans, atmosphere or possibly even the sun.

Future investigations of this part of the hum will prove challenging, as "this is a very small signal that is hard to measure, and the excitation is probably due to multiple interactions in a complex system," said researcher Rudolf Widmer-Schnidrig, a geoscientist at the University of Stuttgart, Germany.

Still, a better understanding of this sound will shed light on how the land, sea and air all interact, he added.

Researcher Dieter Kurrle and Widmer-Schnidrig detailed their findings March 20 in the journal Geophysical Research Letters.