In the Southern Hemisphere summer of 1997, a team from the National Oceanic and Atmospheric Administration (NOAA) was monitoring underwater volcanic activity in the South Pacific when it picked up something it had never heard before.
The signal had an extraordinarily low frequency and unusual intensity, fluctuating roughly once per minute. Its timbre and range were unlike anything in the agency’s records. When researchers sped it up 16 times to bring it within the range of human hearing, it sounded like — well, like a bloop.
That became its name. Bloop.
NOAA’s submarine hydrophone network regularly captures sounds of uncertain origin — signals researchers have nicknamed Upsweep, Julia (for its resemblance to a woman’s voice), and Sea Train, among others. But Bloop stood apart from all of them.
What struck specialists most was the distance between the hydrophones that had detected it: more than 2,000 miles apart.
Very few ocean sounds register across the agency’s entire sensor network. The sheer scale of the detection raised questions about its origin from the outset.
There was no shortage of theories.
Why People Suspected a Sea Monster
Bloop’s rapid frequency variation bore a passing resemblance to the calls of marine animals, and that similarity was enough to fuel speculation about an unknown creature of extraordinary size.
What kept the theory alive was the sheer volume of the sound.
The signal far exceeded the acoustic range of any known animal, including the blue whale, whose call can be detected up to roughly 1,000 miles away — a fraction of the distance Bloop traveled.
That was enough for the idea of a sea monster to take hold in the public imagination, a narrative that persisted for nearly a decade while NOAA scientists continued analyzing the signal.
Bloop became a touchstone within cryptozoology — the study of animals whose existence hasn’t been proven — alongside creatures like the Loch Ness Monster.
Naturalist and cryptozoologist Loren Coleman was among the most vocal proponents of the sea-creature theory. He later conceded, however, that Nessie wasn’t real — and that the sounds associated with the loch had a geological explanation.
How NOAA Traced the Sound to Its Source
Internally, the agency had been pursuing a different hypothesis from the start.
In 2001, a NOAA oceanographer proposed that the signal might be coming from ice along the Antarctic coastline, though he couldn’t yet prove it.
The breakthrough came when the research team repositioned its listening instruments.
By moving hydrophones closer to Antarctica, scientists repeatedly recorded low-frequency, high-amplitude signals that matched Bloop’s acoustic fingerprint.
The pattern repeated often enough to rule out any biological source.
In 2005, NOAA officially concluded that the sound was consistent with a glacial earthquake — or icequake: the fracturing and calving of large icebergs from Antarctic glaciers.
When ice fractures under stress, it releases low-frequency sound waves that can travel vast distances across the ocean before losing intensity.
The evidence continued to accumulate. Between 2007 and 2008, hydrophones deployed in the Scotia Sea detected numerous glacial earthquakes with spectrograms nearly identical to Bloop’s, and those signals let researchers acoustically track the calving of iceberg A53a near South Georgia.
According to NOAA data, tens of thousands of glacial earthquakes occur each year as sea ice and glaciers crack and break apart — a phenomenon the agency says is growing more frequent as global temperatures rise.
Today, we know that Bloop was not a sea monster.
But the fact that it was an early signal of climate change makes it more unsettling than any creature lurking in the deep.
As long as global temperatures keep rising, the sound of Bloop will remain a warning — one we can’t afford to ignore.
