AUDIE CORNISH, HOST:

For decades in the remote icy waters off the coast of Antarctica, people have been detecting a mysterious quacking sound. Crews on submarines heard it first. They called it the bio-duck. Now researchers writing in the journal Biology Letters claim to have identified the source of the noise.

As NPR's Geoff Brumfiel reports it comes from something a lot bigger than a mallard.

GEOFF BRUMFIEL, BYLINE: Submarines on patrol in Antarctica first heard a mysterious sound in the 1960s.

(SOUNDBITE OF QUACKING)

DENISE RISCH: It's really almost like a quack sound. So it goes: Quack, quack, quack, quack.

BRUMFIEL: Denise Risch is a marine biologist for the National Oceanographic and Atmospheric Administration. The quacks were regular and they went on for hours.

RISCH: It has this almost mechanical feel to it.

BRUMFIEL: Some thought it was 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.

RISCH: The sound is very intense. It's very loud. So the thought was it's probably a larger animal producing the sound.

BRUMFIEL: As researchers gathered more data, a suspect emerged: The Antarctic minke whale. Not much is known about this particular whale.

RISCH: It's typically solitary. You don't often find them in large groups. And they're unique in that they're mainly found in ice covered waters. That makes them quite hard to study too and that's also part of the reason why the signal has not been identified earlier.

BRUMFIEL: But last year, during the Antarctic summer, a team from Duke University was studying these whales' behavior. They attached an instrument package to the whale using suction cups. On board was a microphone.

(SOUNDBITE OF MUFFLED QUACK)

BRUMFIEL: Briefly, in one of the recordings, was a muffled, up-close version of the quack.

(SOUNDBITE OF PULSING QUACKS)

BRUMFIEL: That may not sound like the same sound to you.

RISCH: No, they don't sound alike. But the pulses are exactly 3.1 seconds apart from each other.

BRUMFIEL: The same as the quacking. The frequency of the noise matches, too. So mystery solved, sort of, because scientists still don't know why or even how these whales quack.

Geoff Brumfiel, NPR News.

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(SOUNDBITE OF MUSIC)

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