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 the summer of 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 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 volcanoes 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. Five other significant unexplained sounds have been named by NOAA: Julia, Train, Slowdown, Whistle, and Upsweep.

• Cryptozoologists have speculated that the Bloop could have been the sound of a giant sea monster, several times larger than the largest animal known, the blue whale, that made a huge noise a few times many years ago but never since.

• Others guess it could have been bubbles from some kind of huge chemical reaction in the seabed.

• Others guess it might have something to do with the interaction of powerful ocean currents.

• In an alternate reality game promoting the movie, the Bloop was linked to the monster from Cloverfield.

• In The Loch by Steve Alten the Bloop is the call of an undiscovered species of giant eel.

• In Fluke by Christopher Moore the source of the Bloop is a living colony known as the "Goo".

• In Frank Schätzing's novel The Swarm the Bloop is the speech of the intelligent species, the Yrr.

• People have pointed out that the location the sound originated from the southern Pacific Ocean, the claimed location of R'lyeh, thus linking the sound to the sleeping Great Old One Cthulhu in the fiction of H.P. Lovecraft.