What Was ‘The Bloop’? How NOAA Solved the Ocean Mystery

In the summer of 1997, deep-sea microphones across thousands of miles of the Pacific Ocean picked up a massive, unidentified acoustic rumble. The sound was louder than any known animal call on Earth, sparking wild theories about hidden leviathans lurking in the abyss. Decades later, marine scientists unraveled the puzzle to reveal an icy natural phenomenon far grander than any fictional sea monster.

The day the pacific shook

Imagine dropping a sensitive microphone into the deepest trenches of the ocean, expecting quiet hums, only to hear a deafening roar echoing across an entire hemisphere. That is precisely what happened in the summer of 1997.

Researchers monitoring underwater audio feeds at the National Oceanic and Atmospheric Administration (NOAA) spotted a bizarre spike on their monitors. The acoustic event was violent, distinct, and shockingly loud. It was recorded by sensors stationed across more than 5,000 kilometers (3,100 miles) of open ocean.

Scientists quickly gave the strange anomaly a playful nickname: The Bloop.

For years, the public wondered: what was The Bloop sound? Was it a biological call from an undiscovered giant, an unmapped volcanic eruption, or something entirely outside the textbooks?

The cold war ears that heard the noise

Spectrogram of an Antarctic cryoseism. Source: NOAA

To understand how scientists caught the sound, you have to look back at the Cold War. During the second half of the 20th century, the United States Navy built a classified undersea surveillance network called SOSUS (Sound Surveillance System). Its primary job was simple: track Soviet submarines moving through the North Atlantic and Pacific by listening to the hum of their propellers.

When geopolitical tensions cooled in the 1990s, the military granted civilian researchers access to parts of this listening network.

NOAA’s Pacific Marine Environmental Laboratory (PMEL) quickly seized the opportunity. PMEL oceanographers designed and deployed the NOAA autonomous hydrophone array SOSUS-augmented system. These battery-powered, moored underwater microphones floated suspended deep in the water column, listening day and night for undersea earthquakes, volcanic eruptions, and migrating whale pods.

The ocean’s natural highway for sound

Why could hydrophones thousands of miles apart hear the exact same noise? The secret lies in a fascinating layer of the ocean known as the SOFAR channel (Sound Fixing and Ranging channel).

• The Physics: Temperature drops as you dive deeper into the ocean, which slows sound waves down. Further down, crushing water pressure begins speeding sound back up.
• The Sweet Spot: Somewhere between 600 and 1,200 meters deep, these opposing forces create a channel of minimum sound speed.
• The Waveguide Effect: Sound waves entering this zone refract back and forth inside the layer instead of escaping to the surface or scattering on the seafloor.

Low-frequency sound waves can travel across entire oceans within the SOFAR channel without losing much energy. When the Bloop fired off, it hit this aquatic superhighway with extraordinary force.

Anatomy of the sound: why it confused everyone

When acoustic scientists triangulated the source of the 1997 signal, the coordinates pointed toward a desolate patch of the South Pacific: 50° S, 100° W. This remote location sits west of the southern tip of Chile.

The signal stood out for three key reasons:

1. Ultra-Low Frequency: The bulk of its acoustic energy sat below 40 Hz, right at the lower limit of human hearing.
2. Rapid Frequency Rise: Over the course of roughly one minute, the pitch drifted upward, creating a distinct curved sweep on frequency monitors.
3. Staggering Amplitude: It was detected simultaneously on multiple autonomous hydrophones over 3,000 miles apart.

In acoustic terms, an organic vocalization typically shows varied modulation and frequency changes, while an earthquake usually looks like a flat, grinding rumble. Because the Bloop displayed an upward frequency sweep, some early researchers wondered if an animal could have generated it.

There was just one glaring problem: the loudest animal on the planet, the blue whale, reaches roughly 188 decibels. A blue whale call can travel for hundreds of miles, but not thousands. For a living creature to produce the Bloop, it would need to be several times larger than any organism known to science.

Why pop culture blamed sea monsters

Whenever science encounters a blank space on a map, human imagination rushes to fill it.

The public seized on the biological hypothesis with immense enthusiasm. Cryptozoology forums, documentaries, and internet blogs ran wild with speculation. Could a surviving prehistoric Megalodon be patrolling the abyss? Did an unknown species of colossal squid possess sound-producing organs we had never discovered?

The 16x speedup trick

The primary culprit behind the monster frenzy was an audio playback trick.

Because the original Bloop sound was so low in pitch, NOAA audio specialists sped the recording up by 16 times so normal human ears could hear its full tonal range clearly. Sped up sixteen times, the low-frequency rumble compressed into a wet, organic “bloop” sound — almost like a bubble popping or an animal gulping air.

When you play the recording back at its real-time 1x speed, that bouncy chirp completely disappears. In real time, the sound is a heavy, droning, tectonic shudder that resembles distant thunder rolling across the horizon.

The cthulhu coincidence

Fantasy fans noticed something even stranger. In 1928, horror writer H.P. Lovecraft published The Call of Cthulhu, describing an ancient cosmic entity sleeping inside the sunken city of R’lyeh at 47° 9′ S, 126° 43′ W.

The Bloop’s calculated origin point at 50° S, 100° W was only about 1,000 miles away from Lovecraft’s fictional coordinates. For horror enthusiasts and monster hunters, that accidental proximity was too tempting to resist.

Glacial calving generating cryogenic noise. Source: Stacy Matte / Getty Images

The icequake discovery: how NOAA closed the case

While monster stories made great headlines, NOAA scientists kept gathering hard data. The definitive The Bloop sound NOAA explanation arrived through systematic field research rather than sudden revelation.

Between 2005 and 2010, seismologist Dr. Robert Dziak and the NOAA PMEL acoustics team deployed arrays of hydrophones much closer to the frozen waters of Antarctica, including the Bransfield Strait, the Drake Passage, and the Scotia Sea.

As soon as hydrophones entered polar waters, the true culprit revealed itself.

The microphones recorded thousands of loud acoustic signals matching the Bloop’s exact spectrogram curve, pitch progression, and duration. These events were not animals at all; they were cryoseisms — seismic ruptures generated by massive sheets of moving ice.

“The frequency and time-duration characteristics of the Bloop signal are consistent, and essentially identical, to icequake signals we have recorded off Antarctica.”

— Dr. Robert Dziak, NOAA Oceanographer and Marine Geophysicist

The final puzzle piece fell into place in early 2008 when scientists tracked the breakup of giant iceberg A53a near South Georgia Island. As the mammoth iceberg cracked and splintered, it blasted acoustic energy into the water column. The resulting spectrogram proved to be an exact match to the 1997 Bloop signal.

The case was officially solved: The Bloop was the sound of a massive Antarctic iceberg fracturing and calving away from a glacial ice sheet.

How icebergs generate massive noise

How does frozen water make more noise than an erupting volcano or a blue whale? It comes down to mass, tension, and rapid energy release.

The mechanics of iceberg calving cryoseism noise involve several distinct physical processes:

• Tensile Fracturing: Ice sheets spanning hundreds of square miles carry enormous internal tension. When cracks propagate through a thick ice shelf, gigatons of structural stress release in seconds, generating high-amplitude acoustic shockwaves.
• Hydrostatic Crushing: When a massive block of ice snaps and plunges into the sea, air trapped under immense pressure inside the glacial ice vents violently, generating explosive shock pulses.
• Seabed Scraping (Grounding): As icebergs drift into shallower polar banks, their deep keels grind against the ocean floor, vibrating the bedrock and broadcasting continuous low-frequency vibrations through the SOFAR channel.

Animals vs Earthquakes vs Ice

To see why the Bloop stood out from typical ocean sounds, compare its acoustic profile against other common marine noises:

Other bizarre sounds in NOAA’s archives

The Bloop is not the only strange acoustic signal NOAA has recorded. During the late 1990s and early 2000s, PMEL hydrophones logged several other mysterious acoustic profiles:

• Julia (March 1999): A 15-second whining rumble heard across the Equatorial Pacific array. It was tracked to a large iceberg that ran aground off Antarctica.
• Slow Down (May 1997): A descending frequency signal lasting around 7 minutes that gradually dropped in pitch as a drifting iceberg scraped to a halt against coastal sea ice.
• Train (March 1997): A steady, rhythmic grinding sound caused by large ice floes rubbing against one another in the Ross Sea.
• Upsweep (August 1991): An ongoing series of rising tones first detected when PMEL started using SOSUS. Scientists traced this to active undersea volcanic activity along the South Pacific Eltanin fault zone.

Almost every enduring acoustic mystery in modern oceanography has eventually pointed back to two natural phenomena: seafloor volcanism or dynamic polar ice.

The reality behind the myth

The story of the Bloop is a masterclass in how science works. What started as an unsettling noise in the dark depths became a gateway to understanding polar dynamics.

While the idea of an ancient sea leviathan made for exciting folklore, the scientific reality is far more meaningful. The Bloop gave oceanographers a powerful acoustic window into the fragile, grinding mechanics of Earth’s polar ice caps — demonstrating that our planet’s natural geological and climatic forces can produce spectacles louder and more wondrous than any myth we could invent.