The Bloop is not a real animal. The ultra-low-frequency underwater sound detected in 1997 by the U.S. National Oceanic and Atmospheric Administration (NOAA) was once a genuine scientific puzzle, but researchers have since matched its acoustic profile to a far less dramatic source: the cracking and calving of Antarctic ice. The explanation is well-supported, though the Bloop’s brief reign as an unidentified ocean mystery gave it a second life in internet culture that has long outlasted the actual scientific uncertainty.
What the Bloop Actually Was
In the summer of 1997, NOAA’s Equatorial Pacific Ocean autonomous hydrophone array picked up an extremely powerful, ultra-low-frequency sound in the remote southern Pacific, roughly 50°S latitude and 100°W longitude, west of the southern tip of South America. The signal was loud enough to be detected on multiple sensors separated by thousands of kilometers. When the sound’s frequency was sped up to a range audible to humans, it produced a distinctive rising tone that someone at NOAA informally dubbed “the Bloop.” That name stuck.
What made the Bloop unusual at the time was a combination of its amplitude and its frequency profile. It was significantly louder than any known biological source in the ocean, yet its rapid frequency upsweep had a shape that, to some analysts, loosely resembled sounds made by living creatures rather than geological events. NOAA initially listed it as an unidentified sound of possible biological origin, which was an honest and cautious classification given the data available in the late 1990s. That tentative label, however, launched two decades of speculation.
Why People Wanted It to Be a Creature
The timing and cultural context mattered. The deep ocean in the late 1990s was, and largely still is, poorly explored. Estimates at the time suggested that more than 95 percent of the ocean floor had never been directly observed. An unidentified sound of enormous power, detected in one of the most remote stretches of open water on Earth, fit neatly into the narrative that something massive and unknown could be lurking in the abyss. The proximity of the Bloop’s coordinates to the fictional sunken city of R’lyeh in H.P. Lovecraft’s stories was a coincidence that fans of the weird were quick to notice and slow to let go of.
The biological hypothesis also had a thin layer of scientific plausibility. Blue whales, the largest animals known to have ever lived, produce calls that can travel thousands of kilometers underwater. Their calls have been measured at average source levels around 189 decibels (referenced to 1 micropascal at 1 meter), concentrated in the 25 to 29 hertz range. Fin whales produce calls at similar intensities across a comparable frequency band.1The Journal of the Acoustical Society of America. Blue and fin whale call source levels and propagation range in the Southern Ocean If known whales could be that loud, the reasoning went, perhaps some undiscovered species could be louder still.
The problem with scaling that argument up is significant. The Bloop’s estimated source level was far beyond what even the largest baleen whales produce. For a biological organism to generate such a signal, it would need to be vastly larger than a blue whale, and blue whales already appear to be near the upper limit of what ocean ecosystems can support. Research into cetacean body size has shown that the maximum size of filter-feeding whales is constrained by the availability of prey across space and time. Filter feeding on dense swarms of small organisms like krill is the evolutionary pathway that allows extreme body size, but even that strategy hits a ceiling.2PubMed. Why whales are big but not bigger: Physiological drivers and ecological limits in the age of ocean giants A creature large enough to produce the Bloop biologically would need a food supply that simply does not exist in the open Southern Ocean. It would also need to have avoided detection by every other monitoring system, research vessel, and satellite survey ever deployed in those waters.
The Ice Answer
By the mid-2000s, NOAA had revised its assessment. The Bloop’s acoustic signature, they concluded, was consistent with a large icequake: the fracturing and calving of a massive piece of Antarctic ice. This was not a speculative guess. It was the product of years of comparing the Bloop’s characteristics with a growing library of confirmed cryogenic signals recorded by the same hydrophone arrays.
Antarctic ice produces an extraordinary range of underwater sounds. When portions of an ice shelf fracture, the resulting acoustic signals are broadband, spanning frequencies from roughly 10 hertz up to 400 hertz, with individual events lasting anywhere from 10 to 30 seconds. Researchers studying the Nansen Ice Shelf recorded hundreds of these cryogenic signals over just a few months using hydrophone moorings deployed seaward of the shelf. The icequakes were distinguishable from earthquakes by their lack of crustal wave arrivals and low-frequency energy below 10 hertz, and by the presence of relatively high-frequency energy above 100 hertz consistent with sources close to the recording instruments.3Frontiers in Earth Science. Hydroacoustic, Meteorologic and Seismic Observations of the 2016 Nansen Ice Shelf Calving Event and Iceberg Formation These signals, when played back at accelerated speed, can sound strikingly organic to the human ear.
The Southern Ocean is acoustically rich in these events. Long-term hydroacoustic monitoring in the Ross Sea has documented distinct seasonal variations in cryogenic sounds tied to the annual freeze-thaw cycle of sea ice, along with signals from icequakes originating near ice shelves. In some locations, katabatic winds blowing sea ice outward generate strong acoustic noise even during austral winter, creating a nearly year-round backdrop of ice-related sound.4Frontiers. Quantifying Soundscapes in the Ross Sea, Antarctica Using Long-Term Autonomous Hydroacoustic Monitoring Systems The Bloop, rather than being a one-off anomaly, fits comfortably into this category of events. A particularly large calving episode on one of the massive Antarctic ice shelves could easily produce a signal of the Bloop’s amplitude and frequency profile.
How NOAA Heard It in the First Place
The hydrophone arrays that detected the Bloop were originally developed during the Cold War for submarine detection. After the fall of the Soviet Union, NOAA repurposed some of this infrastructure for scientific monitoring of the ocean’s acoustic environment. The sensors are anchored at a depth where sound travels with exceptional efficiency, a layer of the ocean where temperature and pressure conditions create a natural waveguide that allows low-frequency sounds to propagate across entire ocean basins with relatively little energy loss. That is why a single event near Antarctica could register on instruments thousands of kilometers away.
These monitoring capabilities have expanded considerably since 1997. The International Monitoring System established under the Comprehensive Nuclear-Test-Ban Treaty operates 337 facilities worldwide, deploying four detection technologies including hydroacoustic stations. While its primary purpose is verifying the ban on nuclear weapon tests, the system also detects and records signals from a wide range of natural and human-caused events: earthquakes, severe storms, volcanic eruptions, and ice-related phenomena.5reposiTUm. CTBTO verification technologies: application for scientific and civil purposes The same infrastructure that would detect an underwater nuclear detonation is also picking up every significant icequake, whale call, and submarine landslide the ocean produces. If the Bloop were truly a repeated biological signal from a massive unknown organism, the global hydroacoustic network would have recorded it again and again. It has not.
Other Mysterious Sounds and Their Resolutions
The Bloop was not the only unidentified ocean sound from that era. NOAA catalogued several others during the same period, each given informal names: “Slow Down,” “Julia,” “Train,” and “Upsweep” among them. Most have since been attributed to ice-related processes or, in a few cases, to interactions between ocean currents and the seafloor. The pattern is consistent. When a sound was first detected and no immediate match existed in the acoustic library, it was classified as unidentified. As the library grew through more years of monitoring, the matches emerged.
“Slow Down,” for instance, was a sound that gradually decreased in frequency over about seven minutes. Its source was eventually traced to the friction of ice scraping along the ocean floor near Antarctica. “Julia” was initially puzzling, but its characteristics also matched large-scale ice processes. “Upsweep” remains somewhat less definitively explained, though even it is now generally attributed to volcanic or hydrothermal activity on the ocean floor rather than to anything biological. The broader lesson from these cases is that the deep ocean is acoustically noisy in ways that scientists in the 1990s were only beginning to catalog systematically.
What Whale Calls Actually Sound Like on Hydrophones
Part of the confusion around the Bloop stemmed from the fact that whale calls and ice sounds can occupy overlapping frequency ranges. Long-term passive acoustic monitoring has now made it possible to distinguish between them reliably. Antarctic blue whales produce distinctive vocalizations centered at about 18 and 27 hertz. Fin whales call at a fundamental frequency in the 15 to 28 hertz range with overtones around 80 to 90 hertz. Leopard seals vocalize at higher frequencies, roughly 200 to 400 hertz, while Antarctic minke whales produce sounds between 100 and 200 hertz.4Frontiers. Quantifying Soundscapes in the Ross Sea, Antarctica Using Long-Term Autonomous Hydroacoustic Monitoring Systems
Experienced analysts can now tell these species apart by their spectral signatures alone, and they can also tell all of them apart from cryogenic signals, which tend to be broader in bandwidth and lack the repetitive, structured patterning of animal vocalizations. The Bloop’s frequency profile, when examined against this accumulated knowledge, does not match any known whale species. It does, however, match the broadband, transient character of a major ice fracture event. The distinction was harder to make in 1997 simply because the acoustic library was thinner. Researchers had fewer confirmed examples of large-scale ice events to compare against, which is why the biological hypothesis lingered as long as it did.
Why the Myth Persists
Even after NOAA publicly attributed the Bloop to ice, the creature narrative has proven remarkably sticky. Part of this is just the nature of internet folklore: the mysterious version of a story spreads faster and wider than the correction. But there is also a psychological dimension worth noting. Humans are predisposed to hear biological signals in ambiguous sounds. Research on auditory pareidolia has shown that when people are primed with certain contextual expectations, they are more likely to perceive meaningful signals in ambiguous audio, including hearing voices or animal-like patterns in what is actually noise or degraded sound.6Applied Cognitive Psychology. Auditory Pareidolia: Effects of Contextual Priming on Perceptions of Purportedly Paranormal and Ambiguous Auditory Stimuli
When someone listens to a sped-up recording of the Bloop after reading that it “might be a giant sea creature,” they are primed to hear something alive. The rising tone sounds like a groan or a call. It sounds like it comes from a throat. That impression is powerful and genuine, but it reflects how human auditory processing works, not what produced the sound. The same recording, presented as “a large piece of Antarctic ice cracking apart underwater,” would likely strike the same listener as sounding exactly like what it is. Context shapes perception to a striking degree.
The Bloop also benefits from a real gap in public knowledge about the deep ocean. People are generally aware that the ocean is vast and poorly explored, which makes it feel plausible that large creatures could remain undiscovered. And that intuition is not entirely wrong for moderately sized species. New species of beaked whales and deep-sea fish are still described regularly. But the jump from “some species remain undiscovered” to “a creature dwarfing the blue whale has evaded all detection” is enormous. The ocean is poorly explored visually, but it is increasingly well-monitored acoustically. A creature that produces sounds as loud as the Bloop would be the noisiest animal on Earth by a wide margin. Hiding acoustically would not be an option.
What the Deep Ocean Actually Hides
The irony is that the real discoveries coming out of deep-ocean acoustic monitoring are genuinely remarkable, even without sea monsters. Passive hydrophone networks are now used to track whale migration patterns across entire ocean basins, detect previously unknown populations of marine mammals in remote waters, and measure biodiversity without ever sending a ship. The Ross Sea monitoring systems alone have identified signals from at least five distinct whale species, including unidentified whale vocalizations that do not match any catalogued species and remain under study.4Frontiers. Quantifying Soundscapes in the Ross Sea, Antarctica Using Long-Term Autonomous Hydroacoustic Monitoring Systems Those unidentified calls are interesting precisely because they are clearly biological in character but do not yet have a confirmed species attribution. They represent real unknowns, modest in scale compared to the Bloop fantasy, but scientifically significant.
The same monitoring systems are also tracking changes in the Antarctic soundscape tied to climate. Seasonal patterns of ice-related noise have shifted as sea-ice coverage and ice-shelf stability change. Cryogenic signals, the same category of sound that produced the Bloop, are themselves a dataset for understanding how Antarctic ice is behaving over time.3Frontiers in Earth Science. Hydroacoustic, Meteorologic and Seismic Observations of the 2016 Nansen Ice Shelf Calving Event and Iceberg Formation A dramatic increase in the rate or intensity of such signals could indicate accelerating ice loss. The Bloop, in this context, was not a hint at a hidden creature. It was a data point in a much larger and more consequential story about what is happening to Antarctic ice, recorded by instruments that were not even designed for that purpose.