giant manta ray, Manta birostris, swimming on top of lava formations in the dive site El Boiler, San Benedicto Island, Revillagigedo islands.

The Truth Behind The Ocean “Bloop” Sound Of 1997: Scientists Reveal Their Groundbreaking Discovery

From time to time, there are unexplained phenomena that capture the public’s curiosity. Whether it’s a UFO, mysterious tracks in the woods, or an unexplained sound at night, it seldom takes long for fear and curiosity to intermingle.

And when that happens, the public’s imagination runs away with them, and colorful theories abound. That was precisely what happened after a strange “bloop” appeared in the Pacific Ocean in 1997. But after years of speculation and wondering, scientists finally figured out what it was.

Not what they were looking for

giant manta ray, Manta birostris, swimming on top of lava formations in the dive site El Boiler, San Benedicto Island, Revillagigedo islands.
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According to the National Oceanic and Atmospheric Administration (NOAA), researchers had set up underwater microphones called hydrophones throughout the southern Pacific Ocean.

The Antarctic hydrophones allowed them to listen for underwater volcanic activity.


The noise that launched a mystery

French diver Eric Blin, a water environment and coastline waste-management and biodiversity expert who is taking part in the 'sea@dvanced sound' project, checks a hydrophone off the coast of the Ajaccio, the capital of the French Mediterranean island Corsica on September 11, 2019.
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But as Wired reported, one day in 1997 saw those hydrophones pick up an extremely loud, ultra-low frequency sound.

This tone was loud enough that hydrophones placed over 3,000 miles apart were able to pick it up.


Nothing they’d heard before

A aerial view of the submerged Fukutoku-Okanoba volcano, forming a new island near Sou
Gallo Images/”USGS/NASA Landsat data processed by Orbital Horizon”.

Researchers logged multiple instances of this loud sound, but its unique characteristics made it hard to describe as anything but the “bloop.”

And for almost a decade, it confounded the world as nothing like it had been recorded before then.


A daunting task

Ian Pritchard, a marine biologist with the UC Davis Bodega Marine Laboratory, returns to shore with non-native European green crabs collected in underwater traps from Seadrift Lagoon in Stinson Beach, Calif. on Tuesday, Aug. 15, 2017.
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Although members of the NOAA’s Pacific Marine Environmental Laboratory (PMEL) were fascinated by the sound and eager to learn its origin, this was no easy task.

That’s because over 95% of the depths of the world’s oceans have yet to be explored by humans. Finding what made the “bloop” was akin to searching for a very loud needle in the world’s biggest haystack.


The “bloop” goes public

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According to Wired, a wide array of colorful theories gained popularity as the public gradually caught wind of the “bloop” in the years that followed the discovery.

And as media reports described the sound as “organic” in nature, those theories only escalated.


The most popular theory

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Given this “organic” framing of the noise, perhaps the most popular theory among the public was that some massive sea creature had caused the “bloop.”

After all, it’s not unheard of for new species to be discovered even decades after this sound was first detected.


The stuff of legends

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According to the NOAA, one of the possibilities people entertained at the time was that the bloop had come from a giant squid.

But for that to be true, it would have to be a squid of unprecedented size that would look like it came from 20,000 Leagues Under The Sea.


Slightly more plausible

Adult blue whale (Balaenoptera musculus) from the eastern Pacific Ocean
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Other theories held a massive whale responsible for the sound. And since the world is home to some truly gargantuan whale species (like the blue whale pictured here), the idea that one was behind the “bloop” didn’t seem completely implausible.

Still, either a blue whale would have to make a noise that scientists had never observed before, or an even larger, undiscovered whale species would need to exist.


The most outlandish hypothesis

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Those unsatisfied with these explanations who nonetheless believed the “bloop” originated from a life form ended up getting creative with their explanations.

For some, the noise was caused by what the NOAA described as “some sea creature unknown to science.”


Only half-kidding

tonemason David Christison works on his piece 'Cthulhu' as he takes part in a stone carving festival at York Minster on August 19, 2018 in York, England.
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As Wired reported, some horror fans were excited to point out that the point of origin for the “bloop” was just over 1,000 miles from where author H.P. Lovecraft envisioned the sunken city of R’yleh.

For those unaware, that city was where the infamous mythical creature Cthulu was supposed to be held. And while most of these horror aficionados weren’t seriously suggesting an escaped Cthulu was causing havoc near the South Pole, it was an attractive thought.


Alternate theories start to surface

Vanguard-class submarine HMS Vigilant, one of the UK's four nuclear warhead-carrying submarines at HM Naval Base Clyde, Faslane, west of Glasgow, Scotland on April 29, 2019.
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While theories that attributed the sound to mysterious sea life were certainly popular, others didn’t feel that a plausible explanation for the “bloop” required the invention of a new species.

For them, it was more likely that researchers uncovered the sonic side effects of a secret underwater experiment by one of the world’s many military forces.


Far more mundane explanations

Lower Saxony, Bensersiel: A woman takes a picture of a crab cutter moored on a quay wall in the harbor in changeable weather.
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According to the NOAA, some explanations put far more common vessels as the source of the “bloop.” For some individuals, ship engines or even the winches on fishing boats could have made the sound.

But considering how far-reaching and loud the sound was, such explanations severely overestimated how noisy either of those devices is capable of being.


Fun speculation but nothing more

Catfishes, known as invasive species and increasing in number in recent years, living with hundreds of plastic waste at the bottom, are seen during the awareness dive of Sahika Ercumen, United Nations Development Program (UNDP)
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When Wired spoke to NOAA seismologist Robert Dziak, he made it clear that nobody tasked with uncovering the mystery behind the “bloop” seriously thought a giant animal was responsible.

He also explained what led the public to believe otherwise.


A misleading edit

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As Dziak told Wired, “What has led to a lot of the misperception of the animal origin sound of the Bloop is how the sound is played back.”

By that, he meant that the noise commonly heard by the public was about 16 times the normal speed of the “bloop’s” original audio file, which made it sound like an animal cry.


A lumbering rumble

Lightning bolts strike One World Trade Center in New York City as it fans out over the Hudson River and Jersey City, New Jersey during a thunderstorm on April 1, 2023, as seen from Hoboken, New Jersey.
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Dziak further explained that when the sound was slowed down to its normal speed, it sounded more like an earthquake or a rolling thunderstorm.

That meant that for NOAA scientists, the most likely explanation was that a sustained natural process was causing the “bloop.” They just had to figure out what it was.


Some well-trained ears

A hydrophone floats in the water off the coast of the Ajaccio, the capital of the French Mediterranean island Corsica on September 11, 2019 as part of the 'sea@dvanced sound' project.
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What made the “bloop” so exciting to researchers was the fact that it’s actually quite rare for the NOAA’s hydrophones to pick up a sound they didn’t recognize.

As Dziak explained to Wired, almost every sound that comes in fits into one of five major categories: Geophysical, anthropogenic, ice, weather, and animals.


Rare and usually inconsequential exceptions

French diver Eric Blin, a water environment and coastline waste-management and biodiversity expert who is taking part in the 'sea@dvanced sound' project, checks a hydrophone off the coast of the Ajaccio, the capital of the French Mediterranean island Corsica on September 11, 2019.
Boris Horvat/AFP via Getty Images

The weather, ice, and animal categories are self-explanatory. But the geophysical category refers to events like underwater volcanic eruptions or earthquakes, while the anthropogenic category has to do with sounds made by ships and other human creations.

In Dziak’s words, “Anything else is usually just some kind of electronic interference with the signal.”


The search was on

Researchers leaving with the zodiac to collect material with detail of the mooring quay almost submerged by high tide and soil material for earthworks in the foreground, on December 27, 2019 in King George Island, Antarctica.
Alessandro Dahan/Getty Images

But since it was unclear which category the “bloop” fit into (if any), PMEL researchers set up more hydrophones in the region it was first detected.

According to the NOAA, those devices weren’t intended to find the source of the sound so much as to study the sounds of underwater volcanos and earthquakes.


Closer to the truth

Tourists and scientists visit the Base Y operating as a museum by the UK Antarctic Heritage Trust (UKAHT), established in 1950 and used by British scientists for 5 years, in Horseshoe Island, Antarctica on February 26, 2023.
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But the closer those hydrophones were placed to Antarctica, the closer researchers came to discovering the true answer to the “bloop.”

And in 2005, the biggest clue to that answer finally came.


Listening closely

Antarctica, Iceberg In Bransfield Strait, King George Island Background.
Wolfgang Kaehler/LightRocket via Getty Images

As Dziak told Wired, researchers were particularly interested in the data they recorded from the Bransfield Strait and the Drake Passage.

Both of these water bodies are near Antarctica’s northwesternmost peninsula, and their sounds were subject to what Dziak called an “acoustic survey.”


A perfect match

View from the Brazilian Navy's Oceanographic Ship Ary Rongel as it goes through the Drake Passage on its way to Antarctica on March 2, 2014.
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The NOAA’s acoustic survey went from 2005 to 2010, and by the time it was concluded, researchers were confident they had heard the same sounds that confounded the world in 1997.

Dziak described the audio they gathered as nearly identical to the “bloop” in terms of how frequent the sounds were and how long they persisted each time they were heard.


The mystery was finally solved

Iceberg off the Antarctic Peninsula, Antarctica
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Unlike when the “bloop” was first discovered, the increased use of monitoring equipment in the area made it significantly easier to determine what was making these distinct noises.

And so, the NOAA discovered that the researchers in 1997 were listening to an icequake.


What is an icequake?

This file picture shows an enormous iceberg (R) breaking off the Knox Coast in the Australian Antarctic Territory on January 11, 2008.
Torsten Blackwood/AFP via Getty Images

According to the NOAA, this term refers to the sounds and vibrations resulting from an iceberg breaking off a glacier.

Since that was occurring during the organization’s acoustic survey, it stood to reason that something similar happened when hydrophones first picked up the “bloop.”


A sleeping giant

Antarctica, Icebergs...
Wolfgang Kaehler/LightRocket via Getty Images

While it was likely satisfying to uncover the source of the “bloop” at long last, that wasn’t all that this acoustic analysis uncovered.

In Dziak’s words, “It became clear that the sounds of ice breaking up and cracking is a dominant source of natural sound in the southern ocean.”


More common than it seems

A picture taken on August 17, 2019 shows an iceberg calving with a mass of ice breaking away from the Apusiajik glacier, near Kulusuk (aslo spelled Qulusuk), a settlement in the Sermersooq municipality located on the island of the same name on the southeastern shore of Greenland.
Jonathan Nackstrand/AFP via Getty Images

Despite how long it took to determine what an icequake sounds like, it’s actually a pretty common occurrence in the world’s polar regions.

As Dziak said, “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.”


Growing more common all the time

Icebergs break off the Vatnajökull Glacier before floating to sea, July 2006. The 8,300-square-kilometre Vatnajökull is as big as all the other glaciers in Europe put together.
Marcel Mochet/AFP via Getty Images

According to the NOAA, these icequakes have become more frequent in recent decades as the global effects of climate change become more apparent.

As glaciers experience more ice melting, icebergs are more likely to break off from them and eventually melt into the ocean.


Putting the wilder theories to bed

Tourists on Ice breaker, the Kapitan Khlebnikov, breaking through broken pack ice, Weddell Sea, Antarctica
David Tipling/Universal Images Group via Getty Images

Although NOAA scientists never considered the undiscovered beast or secret military experiment theories plausible, this discovery made them even more unlikely.

Considering how well the icequake sounds matched the “bloop,” there was only a remote possibility that it could have come from any other source.


Suspected all along

Rick Jacobsen(cq), foreground, and Larry Combs(cq), space weather forecasters at the National Oceanic and Atmospheric Administration, study recent sun flare activity that appears Earth-directed.
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One revelation that Dziak shared with Wired was that while the PMEL team who uncovered the “bloop’s” origin were eager to do so, the eventual answer didn’t exactly shock them.

Indeed, the best guess at the NOAA for years before that confirmation came was that naturally breaking ice was responsible for the noise.


Still fascinating to learn about

Greenland, Icebergs Break Of The Glacier At Discobay,...
Wolfgang Kaehler/LightRocket via Getty Images

Although it may have been disappointing for some to learn that the truth was a little less fantastical than their theories, there’s always some real satisfaction in solving a mystery.

And barring an unexpected development in the future, this is one mystery that can be considered solved.


PMEL’s work continues

Picture dated 15 January 2004 shows one of the four new hydrophone buoys in the search for the black boxes of the Flash Airlines charter plane which crashed shortly after take-off from Sharm El-Sheikh and killed all 148 people aboard.
Eric Feferberg/AFP via Getty Images

Although it doesn’t sound like they have many decade-spanning mysteries to solve, PMEL’s Acoustics Program continues developing technologies to better capture and analyze long-term data sets indicating sonic changes in the ocean.

If the story of the “bloop” makes anything clear, it’s that those sounds can be surprisingly eloquent regarding how both humanity and natural processes can affect marine environments.

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