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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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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?
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
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
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
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
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
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
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
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.