The Earth Hums, Even Under the Sea

It may look still and quiet, but there's an ever-present hum on the ocean floor. Damsea/Shutterstock

Beneath all the hubbub of life, the Earth is constantly humming.

We made an attempt to record it in 1959, but it took another 40 years to confirm that the Earth is always emitting a background noise, even when there aren't any earthquakes, which is typically when the Earth "starts to oscillate with its resonant frequencies like the clang of a bell."

This hum, or continuous free oscillations as the scientists refer to it, is the noise or vibration of the Earth expanding and contracting, free of any other noise, like those loud and pesky earthquakes. We've confirmed that these oscillations are happening, at least on land, but 70 percent of the planet is covered by water, and we need to know what's happening there, too.

Now we do, thanks to an international team of scientists who have successfully recorded the Earth's hum from the ocean's depths.

Noises off

Active volcanic hot spots are a useful tool for studying Earth's noises, but there are also some drawbacks. Cai Tjeenk Willink/Wiki Commons

To detect the Earth's hum under the ocean, the researchers relied on 57 seismometer stations located on the floor of the Indian Ocean. These stations were focused on a study regarding volcanic hot spots, but the data they collected also lead researchers to the oscillations.

The team selected the two stations with the best data quality and began the process of removing any sounds recorded by the underwater stations. This meant removing any earthquake noise, waves, currents and, naturally, electronic beeps, boops and glitches. The result was the noise equivalent of a dead silent seismometer station that operates on land.

Oh, and the low-frequency hum of the Earth. That was there, too.

The natural vibration of the planet peaked at multiple frequencies between between 2.9 and 4.5 millihertz. Sadly, there's no actual audio recording of this hum because it's around 10,000 times below the threshold of human hearing, which is 20 hertz.

To double-check their work, the scientists compared their sound to land-based observations of the hum. The ocean floor recordings had a similar amplitude as the recording taken by a station in Algeria.

Having more evidence of this hum from the ocean is important for two reasons. The first, obviously, is that by knowing that we have evidence of it in the ocean, we can better determine the source of it. Theories abound about potential causes, from ocean waves to atmospheric turbulence. But in short, we just don't know yet.

The second reason is that this hum could give more insight into the Earth's interior. We normally study the interior of the Earth using seismic waves generated by earthquakes. This method, however, is only applicable in areas prone to earthquakes on a fairly regular basis. If we can use this constant hum as a source of seismic waves, we can begin to map more of the Earth's interior without having to wait for the "clang of a bell" caused by earthquakes.