# In 1997 hydrophones 2,000 miles apart heard a sound across the Pacific so powerful it became the Bloop and years of Antarctic recordings eventually linked its acoustic signature to immense icebergs cracking and breaking apart

> NOAA's account of the 1997 Bloop begins with a listening network that captured remote acoustic evidence. Researchers using underwater microphones in the southern Pacific caught an exceptionally strong low-frequency sound on instruments separated by more than 3,219 kilometers. The recording soon gained...

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Byline: ARGO.net Editorial Team
Published: 2026-07-23T23:36:27+00:00
Categories: Explainer, Oceans

![Massive iceberg floating in Antarctic waters](https://www.argo.net/wp-content/uploads/2026/07/Antarctica_iceberg_ocean.jpg)

[NOAA's account](https://oceanservice.noaa.gov/facts/bloop.html) of the 1997 Bloop begins with a listening network that captured remote acoustic evidence. Researchers using underwater microphones in the southern Pacific caught an exceptionally strong low-frequency sound on instruments separated by more than 3,219 kilometers. The recording soon gained a memorable name. Years of Antarctic acoustic work later gave NOAA scientists a well-supported explanation: a large **Antarctic icequake** had produced the signal.

The episode still matters because the ocean carries sound far beyond the place where it begins. A distant crack, fracture, or collision can leave a pattern on several sensors. That gives scientists a way to investigate remote ice and seafloor activity without being beside it. The Bloop also shows why an arresting sound needs patient comparison with later recordings before anyone treats a first explanation as a settled identification.

## A Pacific listening array catches an unfamiliar signal

In 1997, researchers were listening for volcanic activity in the southern Pacific when the sound appeared. Their **hydrophones**, which are underwater microphones, detected it at stations more than 2,000 miles apart. The array's evidence took the form of acoustic measurements. It recorded how the sound arrived, how its energy changed over time and how its character compared with other ocean noises.

That distance is central to the story. A sound recorded at widely spaced stations can be strong at the source, exceptionally well suited to travel through seawater, or both. Sound travels much faster in seawater than in air. Layers of the ocean can also guide some low-frequency sound for very long ranges. Scientists use differences in arrival time and direction to narrow the region where a signal likely began.

The Bloop became famous because its pattern stood out from familiar calls, engines and short local noises. Curiosity quickly filled the gap between detection and explanation. NOAA's later public account lists several ideas that circulated at the time. Ships and military activity appeared alongside whales and unknown animals. Those possibilities remained public speculation. The later identification depended on acoustic comparisons.

A hydrophone array offers a different kind of observation from an expedition that visits one point in the sea. It can listen continuously and preserve an event for later comparison. PMEL develops long-term acoustic records to study natural processes and human activity in the marine environment. That archive becomes especially valuable when a signal has no immediate visual observation attached to it.

## The shape of a sound can reveal its source

Scientists often examine a sound with a **spectrogram**, a display that shows frequency, strength and duration. It works a little like a map of the recording. A whale call, a ship, an earthquake and breaking ice can each leave different features. The match is rarely based on a nickname or a single loudness measurement. It depends on repeated patterns and on where the sensors indicate the sound came from.

![NOAA spectrogram of the 1997 low-frequency sound known as the Bloop](https://www.argo.net/wp-content/uploads/2026/07/51744_1.jpg)

For the Bloop, NOAA's [Pacific Marine Environmental Laboratory](https://www.pmel.noaa.gov/acoustics/sounds/bloop.html) says the broad-spectrum signals from the summer of 1997 are consistent with **icequakes** generated as large icebergs crack and fracture. The laboratory describes one kind of calving signal as short and broad band, with frequencies from 1 to 440 hertz. Crack growth inside the ice helps generate that wide spread of acoustic energy.

That wording is useful. It supports a physical explanation while keeping the limits visible. The recording was made far from the suspected source, leaving the individual iceberg unseen. PMEL uses arrival direction to identify a likely source area. Its description places the most likely origin between the Bransfield Strait and the Ross Sea, with Cape Adare also identified as a known source of cryogenic signals.

## Later Antarctic recordings supplied the match

Evidence grew as **PMEL** placed hydrophones closer to Antarctica to monitor earthquakes, volcanic activity and ice-related sound. In the Scotia Sea, the instruments detected many icequakes whose spectrograms closely resembled the Bloop. A useful comparison came from modern recordings of icebergs cracking and breaking apart under conditions scientists could study in more detail.

One example involved **iceberg A53a**, which PMEL says its researchers acoustically tracked while it disintegrated near South Georgia Island in early 2008. That work supplied a library of real iceberg sounds and a practical test of the earlier interpretation. Repeated similarities between those signatures and the Bloop made the icequake explanation far stronger than a story about an unobserved giant creature.

Ice can produce several kinds of ocean sound. A calving event can occur when a large section separates. An iceberg that rubs on the seafloor can generate a different, more sustained signal. PMEL's record of a separate 1997 sound called [Iceberg Grounding](https://www.pmel.noaa.gov/acoustics/sounds/train.html) illustrates that distinction. Scientists classify the sound by its pattern and setting, then compare it with known processes.

## A sound can travel across an ocean basin

Large Antarctic ice events can be heard at remarkable distances. PMEL reports that **icequakes** can have enough amplitude for detection on multiple sensors more than **5,000 kilometers** away. Only some cracks produce signals that reach sensors across an ocean basin. Signal strength, background noise, water conditions and the location of the sensor all affect what arrives in a recording.

Distance also makes context essential. Ocean sound comes from weather, animals, seismic activity, vessel traffic and ice. Monitoring systems need long records so scientists can tell an unusual event from a seasonal pattern. A NOAA study of Antarctic icebergs found that their breakup can be a significant natural part of underwater background noise across broad areas of the ocean, including places far from the ice itself.

That research matters for more than solving old mysteries. Researchers who want to measure human-made noise need to understand the natural baseline first. In Antarctic waters, seasonal ice movement and iceberg breakup can change the soundscape substantially. NOAA's [iceberg acoustics work](https://www.pmel.noaa.gov/acoustics/featured-publication/antarctic%E2%80%99s-siren-call-unexpected-effect-iceberg-breakups) connects satellite observations of iceberg volume with hydrophone records from different ocean basins.

Scientists also use listening records alongside maps, weather observations, satellite imagery and direct field measurements. Each method answers a different question. A hydrophone can show when a distinctive sound reached a sensor. Other observations help test what process could have produced it. The Bloop became understandable through that combination of remote listening and later Antarctic comparisons.

## What the Bloop reveals and where evidence stops

Today, the Bloop is best understood as a historic case in **passive acoustic monitoring**. Its key lesson is the growing value of remote records as scientists collect comparable signals, improve their instrument coverage and test explanations against physical evidence.

Available evidence supports a regional, process-level interpretation. Details such as one particular iceberg's size, fracture path and exact moment of breakup remain unknown. PMEL's description gives a likely region and finds the 1997 signals consistent with large iceberg fracturing. That careful level of confidence explains why the sound had such a powerful, unusual character.

Modern programs continue listening because Antarctic ice is scientifically important in its own right. NOAA's [Ross and Amundsen Seas project](https://www.pmel.noaa.gov/acoustics/pmel-theme/ross-and-amundsen-seas) uses moored hydrophones to monitor icequake activity near several ice shelves. The aim is to improve understanding of the **Antarctic soundscape** and ice-shelf stability. The Bloop remains an early reminder that a microphone lowered into the ocean can detect changes far beyond the horizon.

Careful interpretation is part of the value of that listening. A memorable label can outlast the early clues that made it famous, while a technical record keeps accumulating context. The Bloop's explanation became convincing through that longer process. It joined an expanding body of observations showing that Antarctic ice can be a loud and scientifically useful part of the ocean's natural acoustic environment.