A mysterious duck-like sound recorded in the ocean around Antarctica has baffled scientists for decades, but the source of the sound has finally been found, researchers say. “In the beginning, no one really knew what it was,” said Denise Risch, a marine biologist at NOAA Northeast Fisheries Science Center in Woods Hole, Mass. Because the sound was so repetitive, scientists first thought it might be human-made, possibly coming from submarines. The noises also occur seasonally, and have been heard simultaneously in the Eastern Weddell Sea off Antarctica and Western Australia. In February 2013, during the Southern Hemisphere’s summer, Risch’s colleagues tagged two Antarctic minke whales (Balaenoptera bonaerensis) off of Western Antarctica with suction-cup tags.