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 several times during the summer of 1997. The source of the sound remains unknown.

The sound, traced to somewhere around 50° S 100° W (Denoted in the picture as “1”), was detected repeatedly by the Equatorial Pacific Ocean autonomous hydrophone array, which uses U.S. Navy equipment originally designed to detect Soviet submarines.

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.” NOAA’s system ruled out its origin as any known man-made sound, such as a submarine or bomb, or familiar geological sounds such as volcanos or earthquakes. While the audio profile of the bloop does resemble that of a living creature, the system identified it as unknown because it was far too loud for that to have been the case: it was several times louder than the loudest known biological sound (blue whale). Five other significant unexplained sounds have been named by NOAA: Julia, Train, Slowdown, Whistle, and Upsweep.

You can see the rest of NOAA’s unexplained discoveries here: