A peculiar quacking sound, first detected over 50 years ago, has been identified as the call of the minke whale.

The source of the noise had evaded scientists for over half a century. Initially the repetitive low frequency sound, recorded by submarines in the Southern Ocean, was thought to be man made. However the noise only occurred between October and December, which seemed to indicate a migrating animal.

Last year, a research team led by Dr. Denise Risch of the US National Oceanic and Atmospheric Administration sailed to western Antarctica, hoping to solve the mystery once and for all. The team attached acoustic monitoring tags to two wild minke whales, allowing them to study the whales’ vocal activity, movements and feeding patterns. Risch obtained over 26 hours of recordings, and acoustic experts agreed that several of the minke whale’s distinct calls matched the “bio-duck“ sound.

It is still to be determined the purpose of this unusual sound, and why the call is only emitted during certain times of the year. Through further analysis of the recordings, however, Risch and other researchers hope to discover more about the behavior of the minke whale.