Here’s what you’ll learn when you read this story:

• In 1997, NOAA researchers recorded a mysterious ultra-low-frequency sound powerful enough to cross the entire Pacific Ocean.
• The sound’s frequency profile matched an animal call, but its amplitude suggested a creature twice the size of a blue whale.
• Scientists spent years chasing the signal closer to Antarctica, listening for clues to its origin through underwater hydrophones.

In the summer of 1997, MTV had Hanson and the Spice Girls on heavy rotation, teens were eagerly logging on to dial-up internet, and millions blotted away tears as they tuned in for the funeral of Princess Diana. Meanwhile, NOAA researchers were hardly paying attention to any of this as they embarked on a voyage to the South Pacific to record sounds indicating underwater volcanic activity. Listening through hydrophones for activity, they heard something else: BLOOP.

The research team, from NOAA’s Pacific Marine Environmental Laboratory (PMEL), picked up an otherworldly, ultra-low-frequency sound they heard through their underwater microphones, loud enough to register on sensors spread across the entire Pacific Ocean.

“It’s unusual when a sound is recorded on all of the sensors we have deployed,” said NOAA Acoustics Program manager Robert Dziak. “If it’s a ship, or a whale, when it makes a sound in the ocean, it isn’t big enough to be recorded all the way across the Pacific. But this sound was recorded on so many hydrophones, so it stood out in our mind as being something unique.”

The sound sent imaginations spiraling. The frequency profile of “the Bloop,” as it came to be called, roughly matched that of a living creature’s call, which immediately raised a staggering question. What could possibly be large enough to produce a sound detectable over 3,100 miles away? A blue whale, the largest known animal on Earth, can be heard across vast stretches of ocean, but the Bloop’s amplitude suggested something far bigger, perhaps twice that size. Earth’s oceans remain mostly unexplored, and scientists believe they still contain many undiscovered species. The coelacanth, for instance, was long believed to have died out 66 million years ago. It was known only through its fossils, until one turned up alive in a fisherman’s net in 1938. If a “living fossil” could hide in the deep for eons, what else might be down there? Some wondered whether an ancient ocean giant like the Megalodon or something even more gargantuan was still hunting in the abyss.

Others plunged into a Lovecraftian darkness. Maybe this was the actual call of Cthulhu, just waiting to rise up out of the sea, tentacles writhing, a notion helped along by the fact that the Bloop’s coordinates placed it roughly 1,500 kilometers from the fictional location of R’lyeh, Lovecraft’s sunken city. More sober-minded speculators pointed to the elusive colossal squid, a real creature that can reach 46 feet in length and has only rarely been observed. The problem is that cephalopods don’t have gas-filled structures like swim bladders, which most marine animals use to produce sound. A squid capable of generating the Bloop would need vocal machinery unlike anything known to biology.

The PMEL researchers, for their part, suspected something more geophysical. The South Pacific is a seismically active region, riddled with tectonic faults and submarine volcanoes. Could the Bloop be a signal from a cryoseism—an ice quake? Glaciers and ice shelves, when they fracture, release enormous bursts of energy into the surrounding water. The frequency and amplitude of the bloop were consistent with this kind of event. But consistency isn’t confirmation, and the researchers needed more data.

Over the following years, PMEL researchers crept closer and closer to Antarctica, listening through their hydrophones, which kept picking up similar noises at the same low frequency and high amplitude. BLOOP, BLOOP, BLOOP.

It wasn’t until 2005 that they were finally able to confirm the source as an ice quake. As glaciers fracture and enormous chunks of ice break off (a process that’s been accelerating with global warming), they make noises that could be mistaken for mythical sea monsters. Broad spectrum sounds eerily similar to the Bloop include calving, the actual cracking of ice, and iceberg harmonic tremors, which are generated when icebergs scrape against the seafloor or each other.

Whichever iceberg was responsible for the BLOOP heard around the world, or at least the Pacific, was never identified. The amplitude of ice quakes makes them detectable on multiple sensors, even at distances over 3,100 miles. NOAA experts figured out that the direction this sound was coming from indicated that said glacier was probably somewhere between Bransfield Straits and the Ross Sea. Also in the Ross Sea, Cape Adare, a cape of black basalt which lies to the east the Antarctic Continent, is known for strange cryogenic noises and could have also been the source. Sounds that qualify as the spawn of the Bloop have also been heard through NOAA hydrophones in the Scotia sea.

So the mystery has an answer, and it isn’t a monster. For the Megalodon seekers, conspiracy theorists, and Cthulhu faithful, and whoever invented the Bloop creature, this obviously wasn’t what they wanted to hear. But the Bloop still tells a story about something massive, ancient, and powerful. It’s just that the creature is a continent of ice, and it’s dying. Even compared to abominations with gaping mouths and writhing tentacles, that may be the greater terror.