Scientists think this might be a sort of "dinner bell" sound for animals that live in the water's depths and travel upward to the surface, and that this might play a strong role in the global cycling of carbon and the food webs of the ocean.

Researchers led by the University of California San Diego (UCSD) are presenting their findings on this at the 2016 Ocean Sciences Meeting in Washington, D.C., on Monday. Basically, as squid, jellyfish, shrimp and fish in general move up and down from deep in the ocean to the surface in feeding, they make a distinct sound and it has been recorded for the first time.

The animals in the study are many of those that live 660 to 3300 feet below the surface, in the mesopelagic zone of the ocean. A huge number of fish, squid and shrimp live there, for instance. Their weight taken together likely adds up to about 10 billion tons -- which means that with so much mass, they play a strong role in cycling carbon from the atmosphere down to the seafloor, said Simone Baumann-Pickering, assistant research biologist, UCSD, in a release.

The mesopelagic zone is very dark. The creatures there migrate to other water levels at dusk to feed, shrouding themselves in the surrounding darkness to hide from predators. At dawn, they return to their deep-water home.

The research team used acoustic instruments to make recordings of a hum at a low frequency that accompanies the animals' movement to the surface, then back to deeper levels in the morning. While the scientists still wonder which animals make the sound, it seems likely to them that small bony fish there are doing it.

The tone is difficult for the human ear to suss out, at three to six decibels louder than the ocean's background sounds. "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," said Baumann-Pickering in the release.

The sound is mysterious so far, but there's one other possibility: "It's known that some fish are considered to be farting," Baumann-Pickering said in an NPR report, "that they emit gas as they change depths in the water column." The fish have swim bladders that control their buoyancy and emit gas.

While we know how whales, dolphins and many marine mammals sound, the noises made by smaller fish deep in the ocean are more difficult to hear and haven't been thoroughly studied by scientists. Learning about it could help scientists to know more about how the ecosystem works and perhaps how it is reacting to climate change, commercial fishing, predators and other issues.

"I think a large array of (marine) animals will show in the next 10 to 20 years that they are capable of producing and receiving sounds." Baumann-Pickering said in the release.

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