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. read more at livescience.com