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In 1997, while searching for underwater volcanoes off the coast of South America, scientists recorded something they couldn't explain: a strange, exceptionally loud noise. They called it "the bloop."

The bloop was one of the loudest underwater sounds ever recorded: hydrophones (underwater microphones) more than three thousand miles apart all captured the same noise. And researchers at the National Oceanic and Atmospheric Administration, which first recorded the bloop, couldn't figure out what had caused it. But they knew it was something special.

"It's unusual when a sound is recorded on all of the sensors we have deployed," said Bob Dziak, the manager of the acoustics program for NOAA. "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 many hydrophones so it stood out in our mind as being something unique."

Click here, or listen below, to find out the truth behind this tantalizing 90s mystery:

The bloop captured the imaginations of people around the world. Theories began to emerge that this was the call of an aquatic dinosaur, megalodon, or an undiscovered sea creature. These theories gained even more steam when NOAA announced that the sound wasn't man-made. It was "possibly biological."

Clearly, they had discovered a monster.

"I hesitate to say these things because I don't think it's very helpful in the science discussion, but it was considered possibly of animal origin and one idea that was floated out there was the idea that it was a giant squid," Dziak said.

Others were convinced it was not a giant squid, but a monster with a squid face: Cthulhu, the mythical creature from H.P. Lovecraft's Call of Cthulhu. Interestingly, the bloop was recorded just 1,500 kilometers from the place where, in Lovecraft's short story, Cthulhu first emerged. This only added more fuel to the fan theory that Cthulhu was calling out from the deep.

It wasn't until 2005 that scientists were able to definitively explain what the bloop was, and where it came from—and they discovered the truth by accident. In our first episode of Science Solved It, Motherboard's new podcast, we explore the lore of the bloop and the scientific process that led to the solution.