More than 80 percent of the world’s oceans remain unmapped and unobserved, according to NOAA. Since the 1990s, a network of hydrophones originally built to monitor nuclear weapons testing has picked up recordings that have puzzled researchers for decades.
Several of the most famous sounds have since been attributed to icequakes, icebergs scraping the seafloor, and other glacial processes, according to Live Science. But others remain genuinely open questions. Scientists named them the way you name things you cannot identify: simply, descriptively, and with a slight unease about what might come next.
Several sounds are still unexplained.
The Bloop
In 1997, NOAA hydrophones picked up one of the loudest underwater sounds ever recorded off the southern coast of South America. The Bloop was detected by two hydrophone arrays nearly 5,000 kilometers apart, meaning whatever produced it was extraordinarily powerful. The sound mimics marine animal patterns but at a volume far beyond what any known creature could produce. NOAA’s most likely explanation is a large iceberg fracturing, though Science Alert notes that debate about its origin continued for 15 years after it was first recorded.
Julia
Also captured in 1997 and named for the vaguely human quality of its tone, Julia was picked up across the entire equatorial Pacific Ocean autonomous hydrophone array on March 1, 1999. NOAA researchers suspect it was a large Antarctic iceberg running aground on the seafloor. The sound lasted approximately 15 seconds and has been described as resembling a woman’s voice.
The Upsweep
Unlike most unexplained ocean sounds, the Upsweep has been present since NOAA began recording underwater audio in 1991, according to NOAA’s records. It is seasonal, generally peaking in spring and autumn, consists of long trains of narrow-band upward-sweeping sounds, and can be detected across the entire Pacific. The location has been traced to an area of active underwater volcanic seismicity. What is producing it within that zone remains unknown.
Slow Down
Recorded on May 19, 1997, in the equatorial Pacific Ocean, the Slow Down descends in frequency over approximately seven minutes, according to NOAA. Played at 16 times normal speed, it resembles a landing aircraft. The Quick Report and NOAA attribute it to a large iceberg that became grounded, though the precise location and mechanism remain imprecise. The sound is notable for the sheer duration of its frequency drop.
The Train
Recorded on March 5, 1997, also in the equatorial Pacific, the Train produces a steady, rhythmic sound that NOAA has attributed to a very large iceberg grounded near Cape Adare in the Ross Sea, Antarctica. According to The Quick Report, the Train has since been recorded in multiple ocean locations, including the Atlantic and the Mediterranean.
The Whistle
First detected in 1997 and named for its high-pitched, sustained tone, the Whistle remains among the least understood of NOAA’s hydrophone captures. Boing Boing places the Whistle alongside the Upsweep as one of the two sounds for which no plausible source has been officially proposed at all. No geological, biological, or mechanical explanation has gained consensus among researchers.
Wrap up
These sounds were captured by a military surveillance network built to detect nuclear tests. That network turned out to be extraordinarily good at picking up something else entirely. The ocean holds phenomena that science cannot yet fully categorize. The recordings are real, the instruments are reliable, and the explanations are still incomplete.
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