Mysterious ultra-low-frequency sound was heard at the most remote place on Earth

Scientists reveal what caused a mysterious underwater sound recorded over 20 years ago.

In 1997, a strange loud, ultra-low frequency sound was picked up by underwater microphones owned by the National Oceanic and Atmospheric Administration (NOAA) at different listening stations over 5,000km apart.

It was the loudest sound ever recorded underwater and had no immediate source - although it was triangulated to a remote point in the South Pacific ocean near the southern tip of South America - and scientists were left baffled over what could've produced it.

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A team of NOAA oceanographers led by Dr Christopher Fox immediately launched an investigation into trying to identify the source of the sound - nicknamed the Bloop.

Dr Fox questioned the US Navy intelligence to check it wasn't a submarine or bomb detonating and after they denied it being from them, he told CNN: "There are a lot of things making noise down there. Whales, dolphins and fish, the rumblings of the Earth."

During their investigation, the group ended up being 'particularly interested in blue whales,' Dr Fox explaining at the time that the team believed the whales - 'the largest animals ever' - had 'a migratory path up and down the Pacific Coast'.

Or perhaps there was an unknown monster lurking in the depths?

PMEL Acoustics Program

However, after the sound was heard again, ultimately Dr Fox resolved they thought it 'may be related to ice calving' and suspected it was 'ice off the coast of Antarctica'.

Carrying on with Dr Fox's hunch, NOAA and Oregon State University seismologist Robert Dziak told Wired.co.uk that he believed the sound was the result of an icequake - yes, an earthquake but with ice.

Comparing the recording of the noise to 'icequake signals [...] recorded off Antarctica,' Dziak said the 'frequency and time-duration characteristics' between the two were 'consistent' if not 'essentially identical'.

The seismologist explains 'the sounds of ice breaking up and cracking is a dominant source of natural sound in the southern ocean,' a fact determined during a survey of the Bransfield Strait and Drake Passage in 2005 until 2010.

Dziak resolved: "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." Check out the sound here:

Indeed, a retelling of the story on NOAA's website states: “It was there, on Earth’s lonely southernmost land mass, that they finally discovered the source of those thunderous rumbles from the deep in 2005.

"The Bloop was the sound of an icequake - an iceberg cracking and breaking away from an Antarctic glacier.”

So, it would seem like it's good news for Nessie, who's time as the most notorious water monster doesn't seem to be in jeopardy just yet.