Seismologists have long known that Earth can oscillate like a planet-sized bell after the shock of an earthquake. More mysterious is why our planet is also oscillating all the time—at low frequencies and barely detectable by instruments.

A new study suggests a surprising answer: waves at the bottom of the ocean.

Long ocean waves can actually travel thousands of miles from coast to coast, hugging the seafloor. The immense pressure of these waves on the ocean bottom generates the oscillation waves that makes the whole Earth resonate, according to computer models described by French scientists in Geophysical Research Letters. These oscillations are very slow, with periods of up to 300 seconds.

A second and better known mechanism explains the faster oscillations. They are the result of waves colliding in the oceans, generating seismic waves with periods of less than 13 seconds.

Together, the authors say two mechanisms account for the tiny seismic waves that continuously rock our planet. Understanding these small oscillations could help seismologists separate signal from noise, picking out ever fainter seismic signatures from earthquakes or nuclear explosions far away. [Geophysical Research Letters,AGU]

Top image: Fabrice Ardhuin