Vast communities of migrating deep-sea marine life are the culprits behind a mysterious, low-frequency humming sound in the ocean, made as the creatures swim to and from the surface at feeding time.
The discovery, made by University of California, San Diego assistant research biologist Simone Baumann-Pickering, answers a long-standing question. The source of the hum has for years vexed marine biologists, as NPR reports. They knew the sound wasn't consistent with whale calls or other marine mammals, such as dolphins, communicating.
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Now, thanks to high-sensitivity undersea audio recordings, Baumann-Pickering says it's animals such as fish, jellies, shrimp, and squid living in what's known as the ocean's mesopelagic zone – a range 200 to 1000 meters (660 to 3300 feet) below the surface – that are behind the sound.
Creatures in the mesopelagic neighborhood live deep down, in a dark world where the sun barely shines and there's not exactly a bounty of food. So each night, with the safety of darkness, they venture up to the surface where food is more plentiful.