The Bloop

A loud underwater sound picked up by underwater sensors in the South Pacific in 1997. NOAA now attributes it to an icequake, the noise of a large iceberg cracking apart.

NOAA's Pacific Marine Environmental Laboratory says the 1997 summer recordings, which cover a broad range of frequencies, match icequakes, which large icebergs make as they crack and break up. Its hydrophones in the Scotia Sea picked up many icequakes with spectrograms close to the Bloop's, and it used them to follow the break-up of iceberg A53a off South Georgia Island early in 2008. The lab says such sounds are strong enough to register on several sensors more than 5,000 km away. NOAA's National Ocean Service states that the 1997 rumble has since been traced to an icequake.

The sensors belonged to the U.S. Navy. CNN reported in 2002, citing a magazine, that Navy listening posts built to track Soviet submarines caught the sound on sensors spaced 3,000 miles from each other. Brian Dunning of Skeptoid gives the range as up to 5,000 kilometres and places the source, by triangulation, near 50°S, 100°W. That is about 1,750 kilometres west of Chile, far from any island, in water averaging around 4,300 metres deep.

Talk of a sea creature is a retelling. CNN ran its 2002 report under a headline about a deep sea monster. Wikipedia quotes the author Philip Hayward, who says later speculation built on an early "hunch" and, by calling it likely, invited guesses about what kind of thing could make such a noise. Hayward adds that over the decade before he wrote, opinion settled on ice fracturing as the cause.