The deep ocean remains one of the planet’s most unexplored environments. In 1997, researchers from the U.S. National Oceanic and Atmospheric Administration (NOAA) monitoring the Pacific depths recorded a powerful, unusual sound. Scientists nicknamed this brief, undulating noise “The Bloop.” It was unlike any known sound source, and the sheer magnitude of its acoustic energy puzzled experts, leading to years of intense investigation.
Analyzing the Sound’s Signature
The Bloop was registered as an ultra-low frequency sound, below the 40 Hertz (Hz) range, giving it a deep, rumbling quality. It was a high-amplitude event, and this tremendous volume allowed it to travel vast distances through the ocean’s deep sound channel.
The sound was detected by the Equatorial Pacific Ocean autonomous hydrophone array, a system of underwater microphones used for monitoring seismic activity and marine mammals. The hydrophones picked up the Bloop numerous times over about one minute. The sound’s power was immense, registered on sensors separated by more than 3,000 miles (over 5,000 kilometers).
Triangulation localized the sound’s origin to a remote area in the South Pacific Ocean, near 50° South latitude and 100° West longitude. This acoustic power indicated an energy release far greater than almost any other known natural or human-made sound in the ocean. The sound’s signature also featured a distinct “rising” frequency pattern, a subtle clue that helped unlock the mystery.
The Mystery of the Unidentified Source
Before its nature was confirmed, the Bloop’s unique characteristics led to speculation about a biological source. The sound’s profile suggested a pattern similar to the vocalizations of large marine organisms, such as massive baleen whales. This similarity initially prompted researchers to consider the possibility that the sound was produced by an animal.
However, the enormous amplitude of the Bloop contradicted this biological hypothesis. For the sound to be heard across hydrophones thousands of miles apart, the organism producing it would need to be vastly larger than any known marine mammal. Even the loudest blue whale calls could not generate the necessary acoustic power to travel such a tremendous distance.
The scientific puzzle captured the public imagination, leading to theories ranging from undiscovered megafauna to deep-sea cryptids. The Bloop was classified as one of the most powerful unidentified sounds ever recorded, forcing scientists to look beyond known biological explanations.
The Scientific Resolution: Icequakes
The definitive answer to the Bloop mystery emerged years later through continued analysis and new acoustic data. NOAA researchers determined the source was not biological or man-made, but a powerful geological event known as a cryoseism, or “icequake.” This natural phenomenon involves the fracturing and movement of massive ice structures.
The sound was produced by a gigantic iceberg cracking and breaking away from an Antarctic glacier. When these blocks of ice calve, the sudden release of internal stress generates a burst of low-frequency acoustic energy. Seawater conducts sound well, allowing the icequake waves to propagate with minimal loss over oceanic distances.
Further investigation near the Antarctic coast revealed that the Bloop’s acoustic signature was nearly identical to sounds produced by the largest ice-calving events. For example, in 2005, researchers tracked the disintegration of iceberg A53a, which produced sounds matching the Bloop’s profile. This confirmed the 1997 sound likely originated from a massive ice mass situated between the Ross Sea and the Bransfield Strait.
The rising frequency pattern noted in the original recording is consistent with the unique acoustic properties of fracturing ice. As a crack propagates or an iceberg shifts, the resulting sound often exhibits this upward-sweeping frequency characteristic. The resolution confirmed that the Bloop was the result of a major glacial collapse event, releasing energy comparable to a small earthquake.