No one knows what the loudest underwater sound actually is -- here's what it sounds like

The loudest underwater sound ever recorded has been a mystery for 20 years and it still hasn’t got a confirmed explanation.

In 1997, the United States’ National Oceanic and Atmospheric Administration (NOAA) recorded the minute-long, low frequency noise that’s since been dubbed the “bloop”.

It came from an area west of Chile’s southern coast – essentially, the middle of the ocean. It was heard by hydrophones – underwater microphones – located about 5000 kilometres apart.

Photo: Youtube/Cara Cusumano

It’s never been heard again.

Dr Christopher Fox, Chief Scientist of the Acoustic Monitoring Project of NOAA’s Pacific Marine Environmental Lab attempted to find out what the sound was.

But scientists couldn’t figure it out, he told The Atlantic for a short film on the sound.

He even took the recording to the US military to see if it was some form of technology he didn’‚Äčt know about. They didn’t know what it was either.

Fox didn’t think the noise was man-made. “We considered every possibility, including animal origin,” he said.

“Other things in nature that make that sound are blue whales, for whatever reason, but very quickly we understood when we looked at the volume of the sound, certainly it was much louder that the loudest animal sound we are aware of.

“To produce a low frequency you have to be something big.”

But he didn’t want to be involved in any discussion of giant sea creatures.

“It’s captivating because we don’t know what it was. I am glad there are still mysteries on earth and in the universe.”

But years later, NOAA scientists said they believed the bloop had been caused by ocean ice.

NOAA and Oregon State University seismologist Robert Dziak told Wired: “… sounds of ice breaking up and cracking is a dominant source of natural sound in the southern ocean.

“Each year there are tens of thousands of what we call ‘icequakes’ created by the cracking and melting of sea ice and ice calving off glaciers into the ocean, and these signals are very similar in character to the bloop.”

That made it “extremely unlikely the sound was caused by an animal”, Wired reported.

You can listen to the bloop, sped up 16 times, here – and compare it to other underwater recordings.

This article first appeared on Stuff.co.nz. See the original article here.

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