Bloop

From Wikipedia: Bloop or The Bloop is a mysterious ultra-low and extremely powerful underwater frequency detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The source of the sound remains unknown. The sound, traced to somewhere around 50° S 100° W (a remote point in the south Pacific Ocean west of the southern tip of South America), was detected several times by the Equatorial Pacific Ocean autonomous hydrophone array. This system was developed as an autonomous array of hydrophones that could be deployed in any oceanographic region to monitor specific phenomena. It is primarily used to monitor undersea seismicity, ice noise, and marine mammal population and migration. This is a stand alone system designed and built by NOAA's Pacific Marine Environmental Laboratory (PMEL) to augment the U.S. Navy SOund SUrveillance System (SOSUS), equipment originally designed to detect Soviet submarines.