" " A constant hum too low for humans to hear resonates from the floor of the ocean. But why? LITTLE DINOSAUR/Getty Images

A parked truck with the engine running. That's the sound some people have compared to the mysterious, non-stop noise which is emanating from our home planet. We've been aware of this phenomenon for decades now, and while the source of the commotion remains unknown, the scientists who study it have just made an important breakthrough.

Here's a quick history lesson. In the 19th century, geologists began to suspect that the earth might be producing a constant hum, one which rings out even in the absence of earthquakes and seismic events. They also reasoned that the noise must be too quiet for our human eardrums to hear. The official name for this drone is "permanent free oscillations." Until somewhat recently, its existence was only theoretical. A team led by seismologist Hugo Benioff did try to detect the signal in 1959. But their efforts failed because, at the time, science did not yet possess any instruments that were sensitive enough to pick up the hum.



Theory became fact with the advance of technology. In 1997, scientists at the Showa Station — a Japanese research base in eastern Antarctica — were finally able to prove that permanent free oscillations really do exist. The good news was announced a year later, when the Showa team published their findings. Since then, numerous other teams have observed the same noise.

Now, for the first time ever, the earth's hum has been recorded using seismic equipment on the ocean floor. This is a big deal because every previous study which has documented the noise did so with land-based instruments.

The achievement was a hard-won prize. Martha Deen is a geophysicist with the Institut de Physique du Globe de Paris ("Paris Institute of Earth Physics"). Under her leadership, an international team reviewed data collected over an 11-month period from 57 seismometer stations on the Indian Ocean's floor. And that was just the first step. Next, the researchers eliminated all forms of audio interference — such as water currents and technical glitches — from the recordings made at two of the stations.

With the deletion of this extra noise, Deen and her colleagues could finally isolate the hum they were looking for. On Nov. 14, 2017, their findings were published in the journal Geological Research Letters.

Why was it so important to record the oscillations with submerged seismometers? As Deen told us in an email, these instruments will broaden our perspective in a way terrestrial tools never could. "Ocean Bottom Seismometers can cover much larger areas [than land-based ones], for the ocean covers 70 percent of our planet," she says. Deen adds that, "by studying the hum signal at places far from land or islands," we can better understand the phenomena.

Maybe one day, we'll even be able to pinpoint its source. Nobody knows exactly how the hum is being made. A few different hypotheses have been put forth. Some geophysicists think it's generated by the ceaseless pounding of ocean waves onto continental slopes. Others believe that it could be the product of atmospheric turbulence and global wind patterns.

But if the second explanation is true, we'd expect the rumbling's amplitude ("loudness") to vary from season to season. Previous studies have claimed this is happening, yet the new research says otherwise.

Deen's group confirmed that the hum's pitch rises and falls, with its maximum volume hitting a frequency of 4.5 millihertz — about 10,000 times softer than the faintest noises our ears can detect. However, according to the team's findings, the amplitude changes don't correlate with seasonality. Thus, Deen and her colleagues argue that atmospheric issues alone cannot account for the existence of permanent free oscillations.

They also think that their research could open the door for future research on the earth's interior. Geologists use a process called tomography to map out the inside of our world. Think of it as a large-scale MRI scan. Deen explains that scientists "invert the recordings" of seismic waves to decipher the makeup of various layers and structures within the planet. Going forward, ocean bottom seismometers — like those used in her recent study — should give tomographers more data to work with. Hopefully, we'll soon have a better idea of what lies beneath our feet.