The Bloop signal sounds like it was made by an animal, but nobody knows what could have done it (Image: Darryl Leniuk / Photographer's Choice / Getty)

IN THE summer of 1997, an array of underwater microphones, or hydrophones, owned by the US government picked up a strange sound. For a minute, it rose rapidly in frequency; then it disappeared. The hydrophones, a relic of cold-war submarine tracking, picked up this signal again and again during those summer months, then it was never heard again. No one knows what made the sound, now known as “The Bloop” (hear it at www.thebloop.notlong.com).

It’s not the only mysterious sound heard in the ocean. In May 1997, hydrophones picked up the “Slowdown” sound. Over the course of about 7 minutes, it slowly dropped in pitch, rather like the sound of an aeroplane flying past (www.theslowdown.notlong.com). Its origin has been only loosely pinned down: it seems to have originated from somewhere off the west coast of South America, and could be heard from 2000 kilometres away.

So what’s behind the strange noises? The Bloop sounds like it might have been created by an animal, but it is far louder than any whale song, so a marine creature that made it would either be bigger than any whale, or a much more efficient producer of sound. The most popular speculation about Slowdown is that it was caused by the break-up of Antarctic ice – which means it might give an indication of climate change.


There is still no consensus, however, and these two mysteries are just a drop in the ocean, according to Sharon Nieukirk of the US National Oceanic and Atmospheric Administration. “We have hydrophones in the Atlantic, the Arctic, off Greenland, in the Bering Sea and in the Antarctic now, and I am constantly amazed at the variety of sounds coming from the sea,” she says. “There are hundreds of mystery sounds.” (Hear more at www.oceansounds.notlong.com.)

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