Unraveling NOAA's Bloop: Mystery of the Deep

Imagine an unexplained sound so loud it could be picked up by underwater microphones more than 3,000 miles apart—a noise so bizarre it sparked rumors of sea monsters, top-secret submarines, and even alien contact. In 1997, the U.S. National Oceanic and Atmospheric Administration, or NOAA, recorded just such a noise in the South Pacific. They called it “Bloop.”

Bloop was first detected by NOAA’s Equatorial Pacific Ocean autonomous hydrophone array, a system of underwater microphones originally developed to listen for Soviet submarines during the Cold War. The sound was massive—so powerful it was picked up across the entire array, covering a range of over 5,000 kilometers. The source was triangulated to a remote patch of ocean at 50 degrees south, 100 degrees west, far from any shipping lanes and thousands of miles from the nearest coastline.

The spectrogram of the Bloop showed a rapid, rising frequency that lasted about a minute. What made it so captivating, though, was that its audio profile resembled that of a living creature—like a whale call, but orders of magnitude louder. In a 2002 interview, NOAA’s Christopher Fox admitted that the sound was a mystery, noting that Bloop was “far more powerful than the calls made by any animal on Earth.” For a time, speculation ran wild: was there a creature bigger and louder than a blue whale lurking at the bottom of the world?

Rumors and internet forums exploded with theories. One popular idea was that Bloop came from a massive, undiscovered sea monster, sometimes even linked to the fictional city of R’lyeh, the sunken home of H.P. Lovecraft’s Cthulhu, because the coordinates weren’t too far off from Lovecraft’s own description. Others wondered if secret military technology or deep-sea volcanic activity could be responsible. Some even suggested an alien presence.

But as the years passed, researchers started piecing together clues. The NOAA Vents Program, which studies volcanic and hydrothermal activity on the ocean floor, compared Bloop’s spectrogram to known icequake events. They discovered that sounds generated by massive ice calving—when giant chunks of glacial ice break off and crash into the sea—could create similar ultra-low-frequency, high-amplitude signals. Oceanographer Yunbo Xie found that certain mechanical processes in ice, like rubbing and ridging, can cause acoustic emissions resembling Bloop, with waveforms altered by the movement of huge ice floes.

Further evidence came in 2008, when the breakup of iceberg A53a near South Georgia Island was tracked by hydrophones. The acoustic signatures matched Bloop’s profile, reinforcing the idea that non-tectonic cryoseisms, or icequakes, were the culprit. The consensus shifted: rather than a monster or a machine, Bloop was most likely the sound of an enormous chunk of Antarctic ice snapping free and plunging into the Southern Ocean.

But Christopher Fox himself pointed out that while the icequake explanation fits the data, the original sound’s power still raises questions. The Bloop was detected by sensors up to 4,800 kilometers apart, meaning it was louder than any known whale or icequake previously measured. That’s what keeps the story alive in lost media circles and internet mystery boards. And while scientists lean toward the ice origin, no one has ever identified the precise iceberg or calving event that caused it. So, the next time you hear a strange noise on the internet, remember—the ocean is still full of sounds we can’t explain, and at least one of them, over twenty years later, remains a little bit unsolved.