Loudest unexplained underwater sound

In the 1960s the US Navy began installing arrays of underwater microphones around the world in order to track the movements of Soviet submarines. In the summer of 1997 a sound was heard which rose in frequency over a minute and was powerful enough to be detected by multiple sensors in the Equatorial Pacific Ocean autonomous hydrophone array. Analysis of the signal suggested it originated far off the west coast of southern South America, giving a range of around 5,000 km. Nicknamed "The Bloop", this sound was picked up several more times that summer and has never been detected since. Its origin remains unknown although some scientists have speculated it was due to ice calving in Antarctica or even some unknown giant marine species. Of all the unidentified powerful undersea noises detected by navy arrays, The Bloop is the farthest from being explained.