The Big, Deep "Bloop"

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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) several times during 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 repeatedly by NOAA's Equatorial Pacific Ocean autonomous hydrophone array (EPOHA), which uses U.S. Navy equipment originally designed to detect Soviet submarines.  You can listen here to a number of underwater sounds recorded by NOAA.

According to the NOAA description, it "rises rapidly in frequency over about one minute and was of sufficient amplitude to be heard on multiple sensors, at a range of over 5,000 km."  the EPOHA system ruled out its origin as any known man-made sound, such as a submarine or bomb, or familiar geological sounds such as volcanoes or earthquakes. While the audio profile of the bloop does resemble that of a living creature, the EPOHA system identified it as unknown because it was several times louder than the loudest known biological sound—that produced by the blue whale.