Each night, billions of marine organisms around the globe travel from the mesopelagic zone -- a dark stretch of ocean ranging from 660 to 3,300 feet deep -- up to the surface to get their fill.
While the feeding frenzy is well documented, scientists for the very first time have captured what the mass migration of fish, shrimp, jellyfish and squid sounds like.
"It's not that loud, it sounds like a buzzing or humming, and that goes on for an hour to two hours, depending on the day," Simone Baumann-Pickering, a marine biologist at the University of California, San Diego and co-author of the new study, said in a release.
The research, presented Monday during the 2016 Ocean Sciences Meeting in New Orleans, suggests the distinct sound is associated with the upward and downward movements of the organisms at dusk and dawn, and "could be serving as a 'dinner bell' for these deep-water organisms."
In other words, the animals might be actively communicating -- a sort of "it's time to go" signal, Baumann-Pickering told NPR. Or, even more bizarre, it could be the steady rumble of fish farting, as species are known to "emit gas as they change depths in the water column," she said.
Although it is not yet clear which species are creating the sound (small bony fish are the most likely suspects), the low-frequency hum is roughly three to six times louder than background noise. Researchers picked it up using sophisticated audio equipment.
Baumann-Pickering and her team say that ultimately the sound could play a key role in ocean food webs and the global carbon cycle, as well as help scientists better understand this mysterious, deep-sea ecosystem.
Together, the animals living in the ocean's mesopelagic zone weigh an estimated 10 billion tons, according to Baumann-Pickering.
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