"Bloop" is the name given to a loud, low-frequency sound picked up several times by US National Oceanic and Atmospheric Administration hydrophones during the summer of 1997. Its source is unknown.
The Bloop sound has a varying frequency considered the hallmark of marine animals, but its volume was much louder than calls given by even the largest whales. The location of the sound was calculated to be about a thousand miles off the coast of Chile, near the oceanic point of inaccessibility, known as Point Nemo.
The Bloop was picked up by hydrophones in the Deep Sound Channel, a special layer of the ocean where sounds can travel for hundreds of miles. The hydrophone system used to pick up the Bloop is a relic of the Cold War, formerly used to detect Soviet submarines, but today is used for oceanographic research. The name of the network is SOSUS, short for Sound Surveillance System.
Oceanographers are divided on whether the Bloop is of biological origin or from some other source. Other unusual sounds have been picked up the the hydrophone system, given names like Train, Whistle, Slowdown, Upsweep and Gregorian Chant. Many of these have been traced to ocean currents or volcanic activity, but some, such as the Bloop and Slowdown, remain unidentified, though only the Bloop has an acoustic signature that makes it look biological in origin.
Being conservative, one oceanographer that talked to CNN for an article on underwater sounds remarked that the Bloop might be the sound of ice cleaving from glaciers in Antarctica, based partially on the fact that it was detected relatively far south. However, this appears to be pure speculation.
Naturally, some laypeople have speculated that the Bloop might be the call of a giant squid, which are known to dwell in the deep sea and grow to gigantic lengths. However, this is pretty much impossible, as squids lack the gas-filled sac necessary to make any type of call. The Bloop could be a massive whale, but this seems unlikely, as whales must surface at least every two hours to breathe, and one would think that such a leviathan beast would have been sighted by sailors or airplane pilots by now.