When scientists first started hearing an odd noise emanating from the depths of the Pacific Ocean a few years ago, they didn't know what to think. The sound (described as a continuous humming or buzzing that only happens at certain times of day) wasn't from typical ocean activity and was too high a decibel level to be the humpback whales' mating calls that were often heard, NPR reports. But at a meeting of ocean researchers in New Orleans on Monday, one marine biologist announced she and her team may have finally hit on the answer: It may be a "dinner bell" of sorts for the huge numbers of fish, shrimp, jellies, and squid that live in what's called the mesopelagic zone—waters from 660 feet to 3,300 feet below the surface—for when they need to head up to the surface to feed every evening, per an American Geophysical Union press release.
Sensitive acoustic devices called hydrophones helped the researchers, led by Simone Baumann-Pickering, to pinpoint the times of day when the low-frequency hums would take place: once when the sun went down as the fish migrated up to the surface, then again at dawn when they headed back into deeper waters. It's not an insignificant number of fish making that vertical trip—they're estimated to weigh about 10 billion tons together—and the sound they make at feeding time may actually be a purposeful form of communication to signal "it's time to go," Baumann-Pickering says, per NPR. Or it could be something far more basic. "It's known that some fish are considered to be farting," she adds, explaining that fish "emit gas" from the swim bladders that control their buyoancy "as they change depths in the water column"—meaning a huge communal flatulence fest might have been happening every day under our noses. (Astronauts on the Apollo 10 mission heard weird sounds on the dark side of the moon.)