Mysterious underwater sound, described as a "Bio-duck quack," has been identified.

Via American Digest, this fascinating discovery:

First detected by submarine 50 years ago, those who heard it were surprised by its quack-like qualities. Since then, the repetitive, low frequency noise has been recorded many times in the waters around the Antarctic and western Australia. Suggestions for its source have ranged from fish to ships. The researchers now say they have "conclusive evidence" that the bio-duck is produced by the Antarctic minke whale.

Here is what the submariners heard: