The Bloop is the name given to an ultra-low frequency and extremely powerful underwater sound detected several times by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The source of the sound remains unknown.
Strangely, the Bloop originated from the southern Pacific Ocean, which is the claimed location of R’lyeh, linking the sound to the sleeping Cthulhu, from the works of H.P. Lovecraft. But that's probably completely unrelated.