The Mariana Trench is one of the most mysterious places on Earth.
Measuring almost seven miles (36,201ft) deep, the trench sits in the floor of the Pacific Ocean and is so difficult to reach that just a handful of people have ventured down there.
So it’s no surprise that unusual sounds coming from the Mariana Trench sparked fears of an alien invasion when they were first recorded back in 2014.
Lasting between 2.5 and 3.5 seconds, the noises were dubbed ‘biotwangs’, yet their source was unknown.
Ten years later, scientists from the National Oceanic and Atmospheric Administration (NOAA) have reanalysed the sounds – and believe they’ve finally revealed the truth.
The eerie noises were first recorded in 2014 by underwater gliders, which were being used to carry out acoustic surveys of the trench.
Lasting between 2.5 and 3.5 seconds, the five-part sounds includee deep moans at frequencies as low as 38 hertz and a metallic finale that pushed as high as 8,000 hertz.
Researchers were initially baffled by the noises.
However, in 2016, a team from Oregon State University (OSU) suggested that they could be a new type of previously unheard baleen whale call.
Speaking at the time, Sharon Nieukirk, senior faculty research assistant in marine bioacoustics at Oregon State, said: ‘It’s very distinct, with all these crazy parts.
‘The low-frequency moaning part is typical of baleen whales, and it’s that kind of twangy sound that makes it really unique.
‘We don’t find many new baleen whale calls.’
Now, scientists have reanalysed the noises using a combination of visual and acoustic survey data – and their findings suggest that the OSU team wasn’t far off.
Rather than being produced by baleen whales, the new study suggests that Bryde’s whales are responsible.
Writing in their study, published in Frontiers in Marine Science, the team, led by Dr Ann Allen, explained: ‘It was assumed to be produced by a baleen whale, but without visual verification it was impossible to assign a species.
‘Using a combination of visual and acoustic survey data collected in the Mariana Archipelago, we determined that Biotwangs are produced by Bryde’s whales.’
Bryde’s whales can be found around the world in warm, temperate oceans including the Atlantic, Indian, and Pacific.
The researchers spotted 10 of the whales swimming in the area, and even recorded nine making the distinctive noises.
However, to prove these whales were indeed the source, the team turned to artificial intelligence.
‘We used a combination of manual and machine learning annotation methods to detect Biotwangs in our extensive historical passive acoustic monitoring datasets collected across the central and western North Pacific,’ they explained in the study.
‘We identified a consistent seasonal presence of Biotwangs in the Mariana Archipelago and to the east at Wake Island, with occasional occurrence as far away as the Northwestern Hawaiian Islands and near the equator (Howland Island).’
The team still doesn’t know why the whales’ calls are so unusual, or why they’re being made in the first place.
However, speaking to Popular Science, Dr Allen suggested: ‘It’s possible that they use the biotwang as a contact call, a sort of “Marco Polo” of the ocean.
‘But we need more information before we can say for sure.’
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