The Earth hums, and scientists have for the first time recorded the sound in the ocean. Using seismometers in the Indian Ocean, researchers picked up on the sound that is way, way too low for mere human ears to hear, reports Live Science. No recording is available, but a Columbia University scientist not involved with the study in Geophysical Research Letters gives National Geographic a rough description: "It's like taking a piano and slamming all the keys at the same time," says Spahr Webb. "Except they're not nice harmonics. They're oddball frequencies." And ultra-low ones: Specifically, the hum is between 2.9 and 4.5 millihertz, and humans can only begin hearing things at about 20 hertz, or roughly 10,000 times higher.
While scientists have long known about the hum, and have previously recorded it on land, the new research could shed more light on the Earth's interior, and perhaps even help map distant planets, reports the Washington Post. The leading theory is that the hum is caused by waves interacting with each other at the bottom of the ocean, which results in vibrations being sent down into the Earth's crust. The process, however, is complex and not fully understood. For instance, "sometimes a wave on a shallow coast somewhere ripples over the rough sea floor and adds its own frequencies to the hum," per the Post. Whatever the exact reason, one thing is clear: the low drone is a constant one. Read more speculation about the hum here.