THE mysterious deep-sea noise has baffled scientists for a quarter of a century, and we still have no idea what causes it.
It has been described as sounding mechanical, motorised and at times even electronic.
Discovered in 1991, the noise became known as the Upsweep and was found to be audible from each end of the Pacific Ocean.
Whales songs and ship vibrations were initial suspects but both were eventually ruled out by perplexed boffins.
The National Oceanic and Atmospheric Administration (NOAA) has described Upsweep as “a long train of narrow-band upsweeping sounds of several seconds duration each”.
The Upsweep you can hear in this extract is played at 20 times the normal speed.
The sound got louder until 1994 and has gradually subsided since, but can still be detected.
The NOAA believes it peaks in the spring and autumn.
Check out the video for Top 10 Scariest Underwater Sounds!