The Bloop: A 1997 Ocean Sound Louder Than Any Whale

In 1997 NOAA recorded an ultra-low-frequency sound picked up by hydrophones over 5,000 km apart in the Pacific.

It was more powerful than a blue whale, the loudest known animal, fuelling sea-monster theories.

The eerie coordinates sat near the fictional sunken city in a famous H.P. Lovecraft horror story.

By 2012 NOAA concluded the source was an icequake: a giant iceberg cracking near Antarctica.

Similar signals later helped scientists track real icebergs breaking apart in the Southern Ocean.