The Bloop

Summary

In 1997 an American hydrophone array recorded a sound from the deep South Pacific so powerful that it registered on sensors nearly five thousand kilometres apart. It was reported for years as the call of an enormous unknown animal, larger than any whale.

It is an iceberg breaking apart. NOAA now records tens of thousands of these every year.

And the eerie quality that made it famous is a playback artefact. The recording everyone has heard is running sixteen times too fast. At its real speed it sounds like rolling thunder, and it does not go bloop.

What is documented

The detection. In the summer of 1997, researchers at NOAA's Pacific Marine Environmental Laboratory were listening for undersea volcanic activity using the Equatorial Pacific autonomous hydrophone array. They recorded an ultra-low-frequency signal of extraordinary power.

The range, which is the interesting part. It appeared on hydrophones separated by roughly five thousand kilometres. Robert Dziak, NOAA's acoustics manager, has noted how unusual that is: almost every sound in the ocean, whether from a ship or a whale, fades long before it reaches sensors that far apart.

Its shape. The signal lasted roughly a minute, peaked below 50 Hz, and had a distinctive rising frequency. That rising sweep is what made people think of an animal, because it resembles the frequency variation in marine mammal calls.

The problem with the animal reading. For a biological source to produce that much acoustic energy, the creature would have to be far larger than a blue whale. It also does not behave like a call: animal vocalisations used for mating or navigation are sustained and repetitive, and this was neither. And in the decades since, nothing of the kind has been seen, stranded, or found.

What NOAA actually thought. This is worth stating clearly, because the legend depends on getting it wrong. Christopher Fox of NOAA, whose remarks about the spectrogram's resemblance to an animal signal are the source of the entire monster story, told CNN in 2001 that he believed the Bloop was most likely ice calving in Antarctica.

He was right, and he said so four years before it was confirmed, and years before the internet decided otherwise.

The confirmation. From 2005, PMEL began deploying hydrophones much closer to Antarctica, to study seafloor volcanoes and earthquakes. They recorded icequakes: the sounds made by large ice masses fracturing, calving from glaciers, and grinding against the seabed.

The spectral signature was the same. Rising frequency, ultra-low, enormously powerful. The only difference in the 1997 event was scale. The findings were assembled between 2005 and 2010, and the identification was stated publicly by Dziak thereafter.

It is not rare. NOAA's hydrophones detect tens of thousands of icequakes in the Southern Ocean every year. The Bloop was not a unique event. It was a large one, recorded by an array that happened to be listening.

The other sounds. Julia, Slow Down and the Train, all recorded by the same programme and all folded into the same folklore, have been explained the same way: ice, moving.

Leading explanations

An icequake. Established. A large iceberg cracking, calving, or scraping the seafloor near Antarctica. The mechanism is understood, the signature matches, and comparable events are recorded constantly.

Seawater is an exceptionally good sound channel. A fracture event of sufficient size radiates energy that travels thousands of kilometres with little loss, which is why the Bloop was heard across the Pacific and why a whale is not.

A large unknown animal. Dead. It requires an organism substantially bigger than the largest animal that has ever lived, producing a sound unlike any animal call, leaving no trace of itself in three decades of a very well-observed ocean.

Current status

Later explained. The registry keeps this record because of what it demonstrates about how these stories are made.

Nothing was covered up. No evidence was ignored. The scientist most often quoted as the source of the mystery had privately, and then publicly, backed the correct explanation before the legend was fully formed. The recording was released, the analysis was published, and the identification was made.

What made the Bloop famous was a sped-up audio file and a sentence taken out of context, and that combination has proved more durable than twenty years of hydrophone data.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.