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 -...