In case you missed it, the Earth hums. Not like you and I, of course, because that would be utter madness. During those rare moments in which earthquakes aren’t rocking our world, oscillations ripple through the crust, and this background vibration has been charmingly referred to as a “hum”.

Scientists are not entirely sure what causes the hum, however, and a range of hypotheses – from atmospheric turbulence and the mass movement of bodies of water to as-of-yet uncertain geological processes – have been touted. Now, a marvelous new study in Geophysical Research Letters has successfully detected and analyzed the hum after sneakily placing some seismic instrumentation on the seafloor.

“Since its discovery, the hum has been observed at a large number of terrestrial stations,” the authors explain in their paper. However, “no one has observed the Earth's hum at an Ocean Bottom Seismometer during periods of seismic quiescence.”

Until now, that is. The team’s recordings suggest the hum is between frequencies of 2.9 and 4.5 millihertz. This means that, at its lowest frequency, the hum causes vibrations at a rate of 0.0029 times per second, which means you only get one vibration every 345 seconds.

As a point of comparison, the average human heart has a frequency of 1.17 hertz, which is more than 400 times higher. The hum, then, is at an unfathomably low frequency, and detecting it is no small feat.  

The hum – technically a type of tremor known as a “microseism” – is often drowned out by a plethora of other natural phenomena, including “hums” created by the migration of marine life.

Determining the frequency of it has actually been considered by some to be a thankless task, but a new team, led by the Paris Institute of Earth Physics, decided to give it a shot. Eschewing a solely land-based detection method, they also looked to the depths of the Indian Ocean.

As noted by a blog post over on AGU, the array of seismic sensors co-opted for their research were originally designed to visualize hot spots – upwelling plumes of superheated mantle material – emerging in the middle of tectonic plates. Picking out the most high-resolution data, removing as much excess noise as possible, and combining it with several subaerial seismographs in Algeria, the team were able to isolate and identify the hum.

Are we any closer to actually understanding what causes the hum? Not really. Although evidence is mounting that it’s likely something to do with the ocean waves themselves, no explanation to date has been sufficient. Saying that, knowing the fairly precise frequency of the hum will certainly help geoscientists narrow it down, so hopefully it won’t remain a mystery for much longer.