The gentle buzz emerging from the heart of the ocean could be the sound fish farting, experts have said.
Captured by hydrophones a few years ago, the recording puzzled scientists who knew it was something different from regular sounds of the ocean and other signals emitted by marine mammals.
At the 2016 Ocean Sciences Meeting held in New Orleans, biologists suggested a few different theories explaining the bizarre noise.
Experts noted that the hum rises and falls with fish migration in the mesopelagic zone, around 660 to 3300 feet below the ocean's surface.
Dr Simone Baumann-Pickering, a biologist at the Scripps Institution of Oceanography, said this could mean that the fish are communicating with each other.
Speaking to NPR, she said they could be "truly, actively communicating — potentially to initiate migration," acting like a signal letting others know that "its time to go."
Alternatively, she also explained: "it's known that some fish are considered to be farting," emitting gas "as they change depths in the water column."
She added: "The gas comes from a swim bladder inside the fish that controls its buoyancy."