Geologists have detected a new continuous seismic signal in the Earth, but its cause is anybody's guess

Listen closely, and you'll hear the Earth humming - in not just one note, but two. The source of this second signal is a mystery.

For around a decade we've known about Earth's quiet "vertical" hum, probably caused by the steady thumping of deep waves on the ocean floor. Now a team in Germany has discovered a second "horizontal" note, too, and nobody knows what's causing this new signal.

Dieter Kurrle and Rudolf Widmer-Schnidrig of the University of Stuttgart in Germany studied 11 years of data from seismometers at four isolated locations in Germany, Japan and China. The seismometers were designed to detect minute horizontal motion in parallel to the Earth's surface. This can be tricky since atmospheric pressure can obscure the results by pressing on the ground nearby, generating a tilt on the seismometers that might be mistaken for a seismic signal.

They found evidence of a "horizontal" signal at all four stations. The signal migrates by around 1 micrometre one way or the other every three minutes or so, and its horizontal orientation distinguishes it from Earth's "vertical" oscillation. (Geophysical Research Letters)

Though certain events seem to amplify this constant signal - such as earthquakes, volcanoes and large storms - the source is a mystery. "Something entirely new to us is causing this hum," says Spahr Webb of the Lamont-Doherty Earth Observatory in New York, who studied the original hum.

"Something entirely new to us is causing this hum. Its frequency suggests something is 'twisting' the surface of the crust."

The frequency of the new signal suggests something is "twisting" the surface of the crust in some way, says Widmer-Schnidrig. When released, the crust swings back and forth because of its elasticity.

Widmer-Schnidrig's best guess to explain the oscillation is that spiral winds created by atmospheric depressions are inducing some kind of shear force on the Earth's crust, but he admits this theory is a shot in the dark.

Others have been looking to the heavens for a reason. At the American Geophysical Union annual meeting last December, David Thomson of Queens University in Canada and Frank Vernon of the Scripps Institution of Oceanography, California, told researchers that the new hum is synchronised with variations in the Sun's magnetic field.

Widmer-Schnidrig is sceptical about this claim. "What physics led to this correlation? No theory that has been put forward so far has predicted or can explain this observation."