For decades, researchers and submarine crews in icy waters off the coast of Antarctica have been picking up a mysterious quacking sound.
The "bio-duck," as its called, has been heard on and off since Cold War patrols picked it up on sonar during the 1960s.
"It goes 'quack, quack, quack, quack,' " says Denise Risch, a marine biologist with the National Oceanographic and Atmospheric Administration. "It has this almost mechanical feel to it."
Some thought it might be a secret Soviet sub. But over time they came to realize it was an animal. It got a name: the "bio-duck." Although whatever was making this sound had to be a lot bigger than a duck.
"The sound is very intense, it's very loud, so the thought was it's probably a larger animal producing the sound," says Risch.
As researchers gathered more data, a suspect emerged: the Antarctic minke whale. Not much is known about this particular whale. It's the smallest of the baleen whales; it's solitary; and it tends to stay very close to dense sea ice.
"That makes them quite hard to study too and that's also part of the reason why the signal has not been identified earlier," Risch says.
But last year, during the Antarctic summer, a team from Duke University was studying the behavior of these whales. Researchers attached an instrument package to one of the whales using suction cups; on board was a microphone. Briefly, in one of the recordings, was a muffled, up-close version of the quack.
"They don't sound alike, but the pulses are exactly 3.1 seconds apart from each other," says Risch.
The same as the quacking.
The frequency of the noise matches too. Risch and her colleagues published their work in the journal Biology Letters.
So, mystery solved. Well, sort of.
Scientists still don't know why or even how these whales quack.
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