Bloop

(LINK)
The Bloop is the name given to an ultra-low frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The source of the sound remains unknown. While the audio profile of the bloop does resemble that of a living creature, the source is a mystery both because it is different from known sounds and because it was far too loud: it was several times louder than the loudest known animal, the blue whale.  The roughly-triangulated origin of the Bloop is approximately 950 nautical miles from the the more precisely-described location of R'Lyeh, a sunken extra-dimensional city written of by H.P. Lovecraft in his famous short story The Call of Cthulhu, wherein the eponymous dead-but-dreaming creature Cthulhu awakens. Even with this distance separating them, they have been frequently linked.