The "bio-duck" has baffled scientists since the Cold War. NOAA thought the low-frequency quacking sound that occurs frequently off the coast of Antartica might have been the by-product of fishing boats, an elusive Soviet submarine, or some mythical polar duck.
"That makes them quite hard to study, too, and that's also part of the reason why the signal has not been identified earlier," NOAA marine biologist Denise Risch told NPR. "It goes 'quack, quack, quack, quack.' It has this almost mechanical feel to it."