The Bloop (1997): The Mysterious Underwater Sound Explained
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The Bloop (1997): The Mysterious Underwater Sound Heard Across the Pacific
Case status: Scientifically explained Year: 1997 Region: South Pacific Ocean Detected by: NOAA underwater hydrophones Phenomenon: Extremely powerful, ultra-low-frequency underwater sound Current scientific explanation: An icequake associated with large Antarctic ice formations Main mystery: What could produce a sound powerful enough to be detected thousands of kilometres away?
What Was the Bloop?
"The Bloop" was the informal name given to an exceptionally strong underwater acoustic signal detected by NOAA's Pacific acoustic monitoring system during the summer of 1997.
The signal was unusual because it contained extremely low-frequency energy and was strong enough to be detected by hydrophones separated by more than 5,000 kilometres. NOAA describes it as a broad-spectrum sound that rose rapidly in frequency over roughly a minute.
The term "Bloop" does not describe what the sound actually sounded like at its original speed. The familiar audio recording circulated online has been sped up 16 times so that humans can hear the extremely low-frequency signal more easily. NOAA's own acoustics archive provides the original recording and a sped-up version.
When Did It Happen?
The Bloop was detected during the summer of 1997.
NOAA's public Bloop page identifies the recording as a summer-1997 event rather than giving a specific calendar day for the Bloop itself. This distinction is important because several other unusual sounds in NOAA's catalogue have specific dates and are sometimes confused with the Bloop.
The safest wording for publication is therefore:
Summer 1997 — NOAA hydrophones detected the acoustic signal later known as the Bloop.
Where Was the Sound Detected?
The signal was associated with a remote area of the South Pacific Ocean, west of the southern part of South America.
The source region was approximately 50°S, 100°W, based on acoustic localization. This was not a photographically confirmed location of an object or creature. It was an estimated source region calculated from the arrival of the sound at multiple underwater listening stations.
That distinction matters: hydrophones can help researchers determine where an acoustic event originated, but the coordinates themselves do not identify what produced the sound.
How Was the Bloop Detected?
The Bloop was detected by an array of underwater listening instruments known as hydrophones.
A hydrophone is essentially an underwater microphone designed to detect pressure changes and sound travelling through seawater.
Oceanographers use hydrophones to study many different phenomena, including:
- Marine animal sounds
- Underwater earthquakes
- Volcanic activity
- Ice-related acoustic events
- Ships and other human activity
- Other low-frequency sounds travelling through the ocean
The Bloop was detected by multiple sensors separated by thousands of kilometres. The fact that the same acoustic event reached widely separated instruments allowed researchers to study its approximate source and behaviour.
Why Was the Sound So Interesting?
The Bloop was unusual for several reasons.
First, it was extremely powerful compared with many familiar biological sounds.
Second, its low-frequency character allowed it to travel enormous distances through the ocean.
Third, its acoustic pattern rose rapidly in frequency, giving the signal an appearance that could be interpreted as somewhat similar to certain biological sounds when viewed as a spectrogram.
A spectrogram is a visual representation of sound showing how frequency and intensity change over time. NOAA's acoustic archive contains spectrograms comparing the Bloop with other underwater sounds.
The combination of unusual frequency characteristics, high amplitude and enormous detection range made the Bloop an intriguing scientific puzzle.
The Initial Mystery
When the signal was first recorded, researchers did not immediately know what had produced it.
This is an important point that is sometimes exaggerated on the internet.
Scientists did not announce that a gigantic sea creature had been discovered.
Instead, they had an acoustic observation whose source had not yet been conclusively identified.
As researchers attempted to understand it, several possibilities could be considered, including geological activity, ice-related processes, human activity and biological sources.
That uncertainty created the foundation for the later legend.
The Giant Sea Creature Theory
The most famous theory was that the Bloop had been produced by an enormous unknown marine animal.
The idea became popular because the signal's frequency pattern could appear somewhat similar to a biological vocalization when represented visually.
The reasoning went roughly like this:
If a sound was extraordinarily powerful and resembled an animal call, perhaps it came from an animal much larger than any known whale.
The theory was especially attractive because so little of the deep ocean has been directly observed compared with land environments.
However, there was no physical evidence of such a creature.
No unidentified animal was photographed producing the sound.
No body, tissue, DNA sample, feeding evidence or repeated biological recording was linked to the Bloop.
Most importantly, later acoustic comparisons provided a much more conventional explanation.
The Giant Whale Theory
One version of the biological theory suggested that the sound could have come from an enormous whale.
The blue whale is already the largest known animal on Earth and is capable of producing extremely powerful low-frequency sounds.
Because the Bloop appeared extraordinarily powerful, some popular discussions speculated about an animal substantially larger than a blue whale.
But this was speculation rather than evidence.
There is no accepted evidence that an unknown whale species produced the Bloop.
The scientific explanation does not require an unknown animal at all.
The Giant Squid or Unknown Deep-Sea Animal Theory
Another popular idea involved a gigantic squid or another unknown deep-sea organism.
This theory was less scientifically convincing because there was no known mechanism showing that a giant squid could produce an acoustic event matching the Bloop.
Again, the existence of undiscovered deep-sea species is scientifically possible in general, but that does not mean an unidentified species produced this particular sound.
There is no reliable evidence connecting the Bloop to a giant squid or another unknown animal.
The Underwater Volcano Theory
Volcanic and geological activity was another possibility considered when scientists were investigating unusual underwater sounds.
The deep ocean contains extensive volcanic regions, and underwater volcanic activity can generate powerful acoustic and seismic signals.
However, the Bloop's acoustic characteristics did not ultimately provide the best match for an underwater volcanic eruption.
Later comparisons with ice-generated sounds offered a stronger explanation.
The Earthquake Theory
Earthquakes can also produce signals detectable by underwater hydrophones.
This made seismic activity another possible category of explanation.
However, the Bloop was ultimately associated with non-tectonic ice-related activity, rather than an ordinary tectonic earthquake.
This distinction is important because an icequake is not simply another name for an earthquake. It refers to acoustic or seismic activity associated with cracking, fracturing or movement of large masses of ice.
The Explosion or Human Activity Theory
Some internet discussions have suggested that the Bloop could have been caused by an underwater explosion, military activity or another human-made event.
There is no credible evidence establishing such an explanation.
The available NOAA material does not identify the Bloop as a military explosion or secret weapon.
For a website article, it is better to describe these ideas as unsupported possibilities that circulated in popular discussions, rather than presenting them as competing explanations with equal scientific standing.
The Icequake Explanation
The explanation that ultimately became associated with the Bloop involves large Antarctic ice formations.
When enormous masses of ice fracture, crack, move or break apart, they can generate powerful acoustic signals.
NOAA's Pacific Marine Environmental Laboratory found that the Bloop's broad-spectrum acoustic characteristics were consistent with icequakes generated by large icebergs as they crack and fracture.
This explanation became much more convincing because researchers were able to record other ice-related acoustic events.
NOAA hydrophones deployed in the Scotia Sea detected numerous icequakes whose spectrograms were very similar to the Bloop. These recordings demonstrated that large-scale ice activity could produce signals capable of travelling enormous distances underwater.
Why Can Ice Produce Such a Powerful Sound?
The ocean is an exceptionally good environment for transmitting low-frequency sound over long distances.
Large ice masses contain enormous amounts of stored mechanical energy. When ice fractures or moves, that energy can be converted into vibrations and acoustic waves.
The resulting signal can travel through the ocean far beyond the immediate area where the ice event occurs.
NOAA notes that icequakes can be detected by multiple sensors at ranges of more than 5,000 kilometres.
This is one of the most important facts in understanding why the Bloop initially seemed so extraordinary.
The huge detection range did not necessarily mean that the source was a gigantic animal.
It meant that a powerful low-frequency acoustic event had occurred in an environment where such frequencies can propagate very effectively.
How Scientists Compared the Bloop With Icequakes
The key to the eventual explanation was not simply guessing that the Bloop "sounded like ice."
Researchers had access to recordings of known ice-related acoustic events.
When these signals were represented as spectrograms, the patterns showed significant similarities to the Bloop.
NOAA's acoustic archive specifically places the Icequake/"Bloop" recording alongside other cryogenic ice sounds, including iceberg calving and grounding events.
This kind of comparison is much stronger scientifically than simply saying that two sounds "sound similar."
Researchers can compare characteristics such as:
- Frequency range
- Frequency changes over time
- Signal duration
- Acoustic intensity
- Spectral shape
- Propagation across different sensors
The resulting comparison supported the icequake explanation.
The 2000s Icequake Evidence
One of the most useful pieces of evidence came from later observations.
NOAA recorded icequakes associated with large icebergs in Antarctic-related waters.
The recordings provided real examples of how enormous ice masses could generate powerful underwater acoustic signals.
NOAA specifically notes that icequakes detected in the Scotia Sea produced spectrograms very similar to the Bloop. These icequake signals were also strong enough to be detected by widely separated sensors.
This gave scientists something they did not have in 1997: a known physical process producing acoustic signals with characteristics comparable to the mysterious recording.
The A53a Iceberg Connection
NOAA also used icequake signals to acoustically track the breakup and movement of iceberg A53a near South Georgia Island in 2008.
The significance is not that A53a was proven to be the source of the 1997 Bloop.
It wasn't.
Rather, the event provided another real-world demonstration that enormous icebergs can generate detectable acoustic signals over very large distances.
NOAA specifically states that icequake recordings were used to acoustically track A53a as it disintegrated near South Georgia.
Was the Bloop Ever Proven to Be a Sea Monster?
No.
There is no scientific evidence establishing that the Bloop was produced by an unknown marine animal.
The famous "giant sea monster" interpretation came largely from the mystery surrounding the original detection and from popular retellings.
The actual scientific record points in a different direction.
NOAA's current acoustic information identifies the signal as consistent with an icequake produced by large ice formations.
Why Did the Sea-Monster Story Become So Popular?
The story had almost everything required to become an internet mystery.
There was:
- A real recording
- A mysterious name
- A remote location
- An extremely powerful signal
- The largely unexplored deep ocean
- An unusual spectrogram
- A period when the source was not immediately known
- And the possibility, however speculative, of an unknown animal
These elements were repeatedly combined in documentaries, websites, videos and social-media posts.
Over time, the original scientific uncertainty was sometimes transformed into a stronger claim: that scientists had discovered evidence of a gigantic creature.
That claim goes beyond what the evidence supports.
The Lovecraft Connection
Another unusual part of the Bloop's internet history is its association with H. P. Lovecraft's fictional mythology.
The estimated Bloop location is relatively close, in a broad geographic sense, to the fictional location of R'lyeh, the sunken city from Lovecraft's 1926 story The Call of Cthulhu.
The connection is purely fictional.
There is no scientific evidence connecting the Bloop to Cthulhu, R'lyeh or Lovecraft's fictional universe.
The geographical coincidence nevertheless helped the Bloop become popular among mystery and paranormal communities.
It should therefore be presented as an interesting cultural coincidence, not as evidence concerning the sound's origin.
Was the Bloop the Loudest Sound Ever Recorded?
This claim needs careful wording.
The Bloop is frequently described online as "the loudest sound ever recorded in the ocean."
That is an oversimplification.
What can be safely stated is that NOAA describes it as an exceptionally powerful underwater sound, strong enough to be detected on multiple sensors thousands of kilometres apart.
Comparing the absolute loudness of different underwater acoustic events can be complicated because measurements depend on frequency, sensor characteristics, distance, propagation conditions and the way the signal is quantified.
For a scientifically responsible article, avoid claiming that the Bloop was definitively the loudest sound ever produced or recorded on Earth.
What Does the Original Bloop Sound Like?
The famous version heard online is not at its original playback speed.
Because much of the signal was below or around the lower range of human hearing, the recording is commonly played 16 times faster than its original speed.
This makes the signal audible to people and gives it the distinctive rising "bloop" character associated with the mystery.
At its original speed, the event is much lower and slower than the version normally heard online.
This is an important detail because some dramatic descriptions of the sound are based on the sped-up version rather than the original acoustic event.
What Did Scientists Actually Know in 1997?
In 1997, scientists knew that:
- An unusual acoustic event had been detected.
- It was extremely powerful.
- It contained very low-frequency energy.
- Multiple widely separated hydrophones detected it.
- Its source could be localized to a remote South Pacific region.
- Its characteristics did not immediately correspond to an obvious known source.
What they did not know at first was the precise physical process that had produced it.
That distinction between "unidentified at the time" and "unexplainable forever" is central to understanding the Bloop.
What Do Scientists Think Today?
The current NOAA explanation is that the Bloop was consistent with an icequake generated by large icebergs or large-scale Antarctic ice activity.
NOAA's acoustic monitoring program specifically groups the Bloop with cryogenic ice sounds and states that the 1997 recordings are consistent with icequakes generated when large icebergs crack and fracture.
This does not mean researchers photographed an iceberg at the exact moment of the 1997 recording and watched it produce the sound.
Instead, the identification is based on acoustic comparison and subsequent observations of known ice-related events.
That is the scientifically accurate way to describe the conclusion.
Why the Case Still Fascinates People
Even though the Bloop has a scientific explanation, it remains fascinating for a simple reason: the original event demonstrates how little we once understood about the sounds travelling through Earth's oceans.
The ocean can transmit acoustic signals over extraordinary distances.
A massive event occurring thousands of kilometres from a listening station can leave a recognizable acoustic signature.
The Bloop therefore tells an interesting scientific story about ocean acoustics, ice dynamics, hydrophone networks and the difficulty of interpreting signals when their physical source cannot be directly observed.
The mystery was real.
The sea monster was not demonstrated.
The scientific explanation is arguably just as interesting.
The Bloop vs. Other Unidentified Ocean Sounds
The Bloop is sometimes grouped together with other unusual signals recorded by NOAA, including names such as Julia, Upsweep, Train, Whistle and Slow Down.
However, these should not automatically be treated as the same phenomenon.
NOAA's acoustic archives contain numerous unusual underwater recordings, and different signals have different proposed or established explanations.
For example, NOAA identifies the sound known as "Julia" with a likely iceberg grounding event.
The Bloop became particularly famous because of its unusual strength and the mystery surrounding its initial source.
What Is Confirmed?
The following parts of the story are well supported:
The sound was real.
NOAA's hydrophone system recorded the event during summer 1997.
Multiple hydrophones detected it.
The signal was strong enough to be detected across a distance of more than 5,000 kilometres.
The source was localized to the remote South Pacific.
Acoustic localization placed the approximate source around 50°S, 100°W.
The signal was extremely low frequency and powerful.
Its characteristics made it unusual compared with many familiar biological and human-made ocean sounds.
The familiar online recording is sped up.
NOAA's archive identifies the recording as being played back at 16 times its original speed for listening purposes.
The signal is consistent with an icequake.
NOAA's acoustic research identifies large-scale ice fracturing as the likely physical explanation.
What Is Not Confirmed?
Several popular claims should not be presented as established facts:
There is no confirmed giant sea creature associated with the Bloop.
There is no evidence that the signal came from a creature larger than a blue whale.
There is no evidence that the sound came from a secret military experiment.
There is no evidence connecting the Bloop with Cthulhu or R'lyeh.
There is no photograph of a creature responsible for the sound.
There was no confirmed sighting of an animal at the estimated source location.
These claims belong to the history of speculation surrounding the case, not to the established evidence.
Why the Scientific Explanation Matters
The Bloop is a useful example of how science handles an unusual observation.
Scientists first detect something unusual.
They document it.
They investigate possible causes.
They compare it with known phenomena.
They collect more observations.
And when a later observation provides a strong physical match, the original mystery can become understandable.
In this case, later icequake recordings provided the crucial comparison.
That is why the Bloop remains an interesting scientific case even though its source is no longer considered mysterious by NOAA.
Conclusion
The Bloop remains one of the most memorable sounds ever recorded by an ocean-monitoring system.
In the summer of 1997, NOAA hydrophones detected an extraordinarily powerful, ultra-low-frequency sound in the South Pacific. Because the signal was unusual and could be detected across thousands of kilometres, its source initially remained uncertain.
The mystery quickly became associated with ideas about enormous undiscovered creatures living in the deep ocean.
But the evidence eventually pointed somewhere much more terrestrial — or, more accurately, much more glacial.
NOAA's acoustic research found that the Bloop was consistent with icequakes produced by large ice formations cracking and fracturing. Similar ice-generated sounds recorded later provided a comparison that helped explain how such a powerful signal could travel across the ocean.
The Bloop was therefore not confirmed evidence of a sea monster.
Its real story is perhaps more interesting: an unusual sound from one of Earth's most difficult environments to study was eventually explained by studying the natural acoustic behaviour of enormous masses of ice.
The ocean mystery became a lesson in how little information a single unexplained signal can provide — and how much can be learned when scientists keep listening.
Primary and Reliable Sources
NOAA Pacific Marine Environmental Laboratory — Bloop / Icequakes
NOAA's official acoustics page contains the scientific explanation, information about the hydrophone detections, comparisons with icequakes and the Bloop recording.
NOAA PMEL — Cryogenic Ice Sounds
This page contains the original Bloop sound, information about its playback speed and other recordings of ice-generated acoustic events.
NOAA PMEL — Cryogenic Ice Sounds
NOAA PMEL — Spectrogram Archive
Useful for viewing spectrograms of the Bloop alongside other underwater sounds, including ice-related signals.
NOAA PMEL — The Bloop: An Underwater Mystery That Took Nearly 20 Years to Solve
A NOAA article discussing the history of the recording and how researchers eventually connected the sound with ice-related acoustic activity.
NOAA Ocean Service
NOAA's educational material provides another official source for the explanation of the Bloop.
Wikimedia Commons — Bloop Audio
A Wikimedia Commons copy of the Bloop audio is available here. The page identifies NOAA as the original source and explains that the commonly heard version has been sped up 16 times.
Wikimedia Commons — Bloop Audio File
Keywords
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