Bloop

Bloop was an ultra-low-frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The sound is consistent with the noises generated by icequakes in large icebergs, or large icebergs scraping the ocean floor, then by 2002 was believed to also be consistent with large marine animals. NOAA believes it has solved the mystery and now thinks the noise was ice-related.

https://en.wikipedia.org/wiki/Bloop