The constant drone at low frequencies, well below the range of human hearing, shows up in seismic measurements but cannot be explained by events such as earthquakes.

Junkee Rhie and Barbara Romanowicz from the University of California, Berkeley blame the hum on stormy oceans.

Writing in the journal Nature today, they suggest rough water sets the Earth's crust shaking, causing the hum. The sound is probably caused by the conversion of storm energy to oceanic waves, which interact with the shape of the ground at the bottom of the sea.

The hum is faint but represents the release of energy, equivalent to an earthquake of magnitude 5.75 to 6.0 It is made up notes in the frequency range of between two and seven millihertz: in musical terms about 16 octaves below middle C.

To pin down the hum's source, the researchers analysed seismic data from ground-monitoring stations in California and Japan from the 60 days in 2000 when there were no earthquakes to drown it out. They worked out the hum was tied to winter in each hemisphere, when ocean storms are most severe. "Our results show that the ocean plays a key role in the excitation of the Earth's hum," they write.

Other scientists blamed variations in atmospheric pressure, because they thought it might be causing a drumming on the Earth's surface. Others thought it was down to earthquakes deep in the Earth, which release energy without rupturing faults.