The Bloop: The Ocean's Loudest Mystery and the Ice That Solved It

2025-05-23 · Signals & Sounds · 2 min read

In 1997, researchers at the U.S. National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory were listening to the ocean through a network of underwater microphones when something extraordinary crossed their screens. It was an ultra-low-frequency sound that rose in pitch over about a minute, and it was staggeringly powerful: hydrophones more than 5,000 kilometres apart picked it up. Its source was traced to a remote patch of the South Pacific, roughly at 50 degrees south, 100 degrees west, far from any shipping lane. Played back at sixteen times its natural speed, the recording made a strange liquid gulp, and the signal earned the name that made it famous: the Bloop.

The listening system itself was a piece of Cold War inheritance. The U.S. Navy had built the SOSUS hydrophone network to track Soviet submarines; after the Cold War, NOAA gained access and added autonomous hydrophones of its own to monitor undersea earthquakes, volcanic rumblings, and whale calls. The scientists who studied the Bloop knew every category of ocean sound, and this one resisted them all. Christopher Fox, the NOAA researcher who oversaw the array, noted that the signal's varying character resembled that of a living creature, with one enormous problem: it was far more powerful than any call made by any known animal, including the blue whale, the loudest animal on Earth.

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