Earth's Hum: Scientists Record the Very Sound of Earth—but Don't Know Why It Happens

Earth, as seen on April 16, 1972, by the Apollo 16 crew. NASA

Scientists have recorded the mysterious sound of the Earth deep underwater for the first time. The eerie hum could be key to understanding the makeup of the planet itself—but no one knows what makes it.

First proposed in the 19th century, scientists have been trying to record the Earth's hum since 1959. The Earth expands and contracts very slightly all the time, creating a steady sound that's inaudible to human ears. No one really knows why this happens, but scientists think ocean waves could be the source.

Violent Ocean Forces

Powerful waves pummel the surface of Earth all the time, so experts think that pounding might fuel its movement. Covering two-thirds of the planet, they are a great place to hunt for the hum. Unfortunately for researchers, however, these vast areas are widely unexplored.

"Station coverage in the oceans is much sparser than on land, leaving great parts of the Earth uncovered," scientists wrote in the recent study, published in Geophysical Research Letters.

In 1998, Japanese scientists successfully proved the sound was real, but it has never been captured beneath the sea before. Previous underwater recordings have been have been thwarted by competing signals from earthquakes and other ocean sounds.

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Waves crash against rocks on July 28, 2011. Steve Shupe/Flickr

Geophysicists in this study used sophisticated "ocean bottom seismometers" to pick up and isolate the Earth's hum. The very sound of the ocean itself, however, made it hard to hear the Earth.

"A low noise level is needed to observe the small signal amplitude of the hum," the researchers wrote. "At the ocean bottom, the noise level at long periods is generally much higher than at land stations."

The team used sophisticated mathematical techniques to remove the sound of ocean currents and electronic glitches.

Acoustic Resonance

Violent ocean waves aren't the only explanation for the sound of the Earth. Something called acoustic resonance might offer an alternative.

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Tuning forks can be used to test acoustic resonance. Adrian Clark/Flickr

When the frequency of one vibrating object matches the natural frequency of another, this second object will vibrate in response. You can test this with two piano tuning forks: Hit one, and the other will visibly vibrate in harmony. Some scientists think the Earth's atmosphere amplifies the sound of the solid ground when it produces the right vibrations.

However, research on volcanic eruptions suggests acoustic resonance may not be enough to explain the Earth's hum.

Whatever the precise cause of the hum, these new recordings can help improve our understanding of the planet itself. We already track vibrations to predict and measure earthquakes. Isolating them beneath the ocean could increase these capabilities.

"The Earth's hum can be used to study the Earth's deep interior," the authors explained. By analyzing the sound of the planet, scientists might one day understand what is really going on beneath the surface.

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