Some 10 billion tons of shrimp, squid, bony fish, crustaceans, and jellies live in the dark waters in the middle of the ocean—between 660 and 3300 feet below the surface, in the mesopelagic zone. Each night, they migrate to the surface of the water to feed, sinking back down before the Sun comes up. 

They don't go quietly, as a pair of San Diego-based marine biologists have found. New recordings (spotted by Gizmodo) illuminate the dull roar that accompanies these daily migrations of what are called deep-scattering layer organisms

The researchers dropped tubes full of sensitive audio equipment into the water off the coast of San Diego to listen in on the creatures as they made their way up and down the water column. They heard noises between 300 hertz and 900 hertz in frequency. At only three to six decibels louder than the background noise of the ocean, the sounds can be detected by finely tuned sensors but are quite faint to human ears.  

“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,” biologist Simone Baumann-Pickering of the University of California, San Diego said in a press releaseHer study was presented this week at the Ocean Sciences Meeting, sponsored by the American Geophysical Union and other scientific organizations.

The significance behind the quiet ocean hum and the creatures behind it are still a mystery. It’s probably made by bony fish, and could be a signal to other fish that it’s time to migrate up to the surface or return to deeper waters. However, it could also lure predators like tuna, birds, and water-dwelling mammals, alerting them that tasty treats are on the move. 

[h/t Gizmodo]