The Bloop was an ultra-low-frequency underwater sound recorded by NOAA in 1997 and once thought to be a giant sea creature. By 2005 NOAA had matched its acoustic signature to a cryoseism, or icequake: a massive Antarctic iceberg cracking and calving into the sea.
Published: June 5, 2026. Last reviewed: June 5, 2026.
What the Bloop actually was
The Bloop was a single ultra-low-frequency acoustic event, recorded by NOAA hydrophones in the summer of 1997, loud enough to register on sensors more than 5,000 kilometers apart across the South Pacific [1][2].
For roughly a minute the signal climbed steeply in frequency, the rising “bloop” you hear when the recording is sped up to audible speed. NOAA’s Pacific Marine Environmental Laboratory logged it through the same acoustic monitoring program built to catch undersea earthquakes and volcanic rumble near mid-ocean ridges [2]. By a wide margin it was the loudest unidentified sound the agency’s Vents Program had on file, which is exactly why it drew a crowd. The sound was real, well-located, and far louder than the surrounding ocean. What it was not, despite a decade of speculation, was alive.
The conflation worth resolving: the Bloop’s discovery and its explanation are separated by years. NOAA caught the sound in 1997, published its hydrophone methods in 2001, and only reclassified the source with confidence after putting instruments closer to Antarctica in the mid-2000s [3][4]. That gap, more than a decade between a sound and a settled cause, is why the Bloop spent so long as a magnet for stranger ideas. As of 2026, NOAA describes the Bloop as consistent with a large cryoseism, the technical term for an icequake [1][2].
How a Cold War listening network caught it
NOAA recorded the Bloop on the Equatorial Pacific Ocean autonomous hydrophone array, a civilian descendant of the U.S. Navy’s SOSUS submarine-tracking network that scientists gained access to after the Cold War ended around 1991 [3][5].
The Equatorial Pacific Ocean autonomous hydrophone array locates events by triangulation. A low-frequency sound reaches widely spaced hydrophones at slightly different times, and those time differences fix the source. NOAA’s Vents Program had started borrowing the Navy’s seafloor arrays for science around 1990, processing the raw audio at a naval facility on Whidbey Island before fielding its own autonomous hydrophones in the open ocean [5]. Christopher Fox and colleagues at PMEL described the array’s seismic performance in a 2001 paper in the Journal of Geophysical Research, reporting it could locate ocean-floor events across the whole equatorial Pacific [4]. The Bloop fell out of that same data stream.
Low frequencies travel extraordinarily far in seawater because the deep ocean forms a natural waveguide, the SOFAR channel, that traps sound and carries it for thousands of kilometers with little loss. That single fact explains the reach. On the math: a sound detectable over 5,000 kilometers does not require a 5,000-kilometer animal, it requires a loud event and a good waveguide. A source near Antarctica could be heard by hydrophones in the tropics, which is exactly why the signal looked stranger than it was [5].
Why so many people thought it was alive
The Bloop sounds organic mainly because the widely shared audio clip is sped up roughly sixteen times, compressing a minute-long, nearly sub-audible rumble into a few seconds of something that resembles an animal call [3][7].
Early coverage leaned into that impression. Christopher Fox, then at NOAA, told reporters the signal rose in frequency the way a living thing’s call might, while stressing that any creature producing it would have to be far louder than a blue whale, the loudest animal on record [3][7]. That caveat usually got cut. What the data ruled out came first: Fox’s team excluded submarines, bombs, and familiar geology such as volcanoes and earthquakes early on, leaving an unidentified but plainly natural source [3]. The clip spread fastest online in the late 2000s, surfacing in the alternate-reality marketing game for the film Cloverfield and on countless message boards as proof of something enormous and unseen [7].
The Cthulhu coordinate coincidence
NOAA triangulated the Bloop to roughly 50°S 100°W, a remote patch of the South Pacific west of the southern tip of South America [3]. Readers of H.P. Lovecraft noticed that his 1928 story “The Call of Cthulhu” places the sunken city of R’lyeh at 47°9’S 126°43’W, and the internet did the rest [3][6]. The two points sit in the same quadrant of the ocean, but they are about 1,900 kilometers apart, roughly the span from New York to central Florida. A shared neighborhood on a map the size of an ocean basin is not a match.
The ice that solved it
NOAA solved the Bloop by listening closer to its source, deploying hydrophones in the Scotia Sea near Antarctica in the early 2000s and recording thousands of icequakes whose spectrograms matched the 1997 signal [1][2].
A cryoseism is a fracture event in ice rather than rock. When a large iceberg cracks, calves from a glacier, or scrapes along the seabed, the sudden release sends a broadband pulse through the water from about 1 to 440 hertz [2]. The motion is violent and brief, which is how an icequake can rival a small earthquake in acoustic energy while producing nothing a tectonic seismometer would log. The 2012 reclassification rested on this match between the Bloop and modern calving noise [3].
The decisive case came from one iceberg. In early 2008 PMEL acoustically tracked iceberg A53a as it disintegrated near South Georgia Island, and the icequakes it threw off carried the same rising, broadband signature as the Bloop [2][3]. NOAA placed the 1997 source somewhere between the Bransfield Strait and the Ross Sea, possibly near Cape Adare, all stretches of Antarctic coast where ice grinds into open water [1][2].
Reading the spectrogram
A spectrogram plots frequency against time, and the Bloop’s reads as a quick upward smear rather than the steady tone an animal would hold. NOAA’s icequake recordings show the same shape, including a harmonic-tremor variant with a 40-hertz fundamental and evenly spaced overtones produced as fractured ice grinds against itself [2]. Set side by side, the 1997 trace and the modern ice traces are the same handwriting.
The Bloop among other unexplained ocean sounds
NOAA’s hydrophones logged at least six famous unidentified sounds across the 1990s, among them Upsweep in 1991, Julia in 1999, and the Bloop in 1997, and most have since been traced to ice or undersea volcanism [2][8].
The Bloop was never alone. NOAA’s Vents Program catalogued a small family of strange recordings in the same decade, and the pattern that emerged is telling: nearly every one resolved into ice friction or a submarine volcano once researchers collected nearby reference recordings. Two of them, Train and Slow Down, share the Bloop’s likely cause, broadband noise from icebergs scraping and grounding against the seafloor, while Upsweep and Whistle point instead toward undersea volcanism in the Pacific [2][8].
| Sound | Recorded | Leading explanation |
|---|---|---|
| Upsweep | 1991 | Suspected undersea volcanic activity |
| Train | March 1997 | Likely Antarctic ice friction |
| Slow Down | May 1997 | Iceberg grounding on the seabed |
| Whistle | July 1997 | Suspected submarine volcano |
| Bloop | Summer 1997 | Icequake (confirmed) |
| Julia | 1999 | Large iceberg grounded off Antarctica |
Slow Down earned its name honestly, gliding downward in frequency over about seven minutes as an iceberg dragged across the bottom [8]. Julia, recorded on March 1, 1999, was loud enough to cross the equatorial array and reads as a large iceberg grounded off Antarctica [8]. Seen together, the Bloop looks less like an outlier than the loudest, best-known entry in a catalog the ocean keeps filling. The lesson repeats: a strange recording usually means a missing reference sound, not a missing creature [2][3].
What the Bloop changed about listening to the ocean
The Bloop matters less as a solved riddle than as a calibration check on how people read unfamiliar ocean sounds picked up by instruments first built to find submarines and earthquakes [4][5].
The same hydrophone networks now catch many thousands of icequakes a year in the Southern Ocean, and the count has climbed as warming speeds iceberg calving [1][3]. The Bloop, once floated as the largest unknown animal on the planet, turns out to be an ordinary member of that enormous chorus, distinctive only because it was recorded before anyone had nearby reference sounds to place it. Each year of recordings makes the Bloop look more ordinary and the Southern Ocean sound busier, and the same data now doubles as a climate measurement, since calving rates track the stress on Antarctic ice [1][3]. We do not yet know every voice in that chorus, which is the honest and interesting part. The fuller record of well-documented but still-open cases sits in Eso Vitae’s science and natural anomalies collection.
That is the useful lesson. A signal can be genuinely anomalous, loud, well-measured, and still resolve into something physical once you gather the right comparison data. The Bloop did not need a monster. It needed a closer microphone.
Felix Chen writes about the edge of measured science for Eso Vitae; more of his work lives in the Felix Chen anomaly files.
Frequently asked questions about the Bloop
What was the Bloop?
The Bloop was an ultra-low-frequency underwater sound NOAA recorded in 1997, later identified as an icequake from a fracturing Antarctic iceberg rather than a marine animal.
When and where was the Bloop recorded?
NOAA recorded it in the summer of 1997 and triangulated the source to about 50°S 100°W, a remote stretch of the South Pacific Ocean.
What caused the Bloop?
A cryoseism, or icequake: a large iceberg cracking, calving, or scraping the seabed near Antarctica, releasing a broadband pulse from roughly 1 to 440 hertz.
Was the Bloop a sea monster or Cthulhu?
No. The creature idea came from a sped-up clip, and the Cthulhu link is a coincidence; R’lyeh’s fictional coordinates sit about 1,900 kilometers from the source.
How was the Bloop detected from so far away?
Low-frequency sound travels thousands of kilometers in the ocean’s SOFAR channel, so a loud Antarctic icequake reached hydrophones more than 5,000 kilometers apart.
Who studied the Bloop?
NOAA’s Pacific Marine Environmental Laboratory, including scientist Christopher Fox, recorded and analyzed it using the Equatorial Pacific Ocean autonomous hydrophone array.
How did NOAA confirm the icequake explanation?
By deploying hydrophones near Antarctica in the 2000s and matching the Bloop to icequakes, including those from iceberg A53a as it broke apart in 2008.
Does the Bloop still happen?
Yes. NOAA’s hydrophones record many thousands of similar icequakes each year in the Southern Ocean, and calving has increased as the climate warms.
