The loudest underwater sound ever recorded remains a mystery 20 years on, with scientists admitting they are still unsure of the exact source.

The sound was captured in 1997 by the United States' National Oceanic and Atmospheric Administration which named the low frequency noise the "bloop". It was heard by underwater microphones – hydrophones – located about 5000km apart with the sound emanating from an area west of Chile's southern coast.

Scientists at the time attempted to identify what the sound was but couldn't figure it out.

Dr Christopher Fox, chief scientist of the Acoustic Monitoring Project of NOAA's Pacific Marine Environmental Lab thought it might have been a form of secret military technology but the US military also didn't know what the sound was either.

Dr Fox doesn’t believe the sound was man-made.

"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," he told The Atlantic.

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

Today, scientists think the sound was 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.