In the summer of 1997, scientists recorded a strange, loud noise originating from an area west of Chile's southern coast. They dubbed it "the bloop".
While searching for underwater volcanoes, researchers at the National Oceanic and Atmospheric Administration recorded the infamous loud, ultra-low frequency sound on hydrophones.
These underwater microphones the US Navy originally developed were 2,000 miles apart in the Pacific Ocean.
The sound, which lasted for about one minute, was one of the loudest underwater sounds ever recorded.
Below, you can listen to the bloop sped up 16 times:
Over the years, theories about the mysterious ocean sound's origin abounded.
Some suspected it was the sound of military exercises, ships, a giant squid, blue whales, or a new sea creature. After all, humans haven't explored more than 80 percent of the world's oceans.
"We considered every possibility, including animal origin," Christopher Fox, chief scientist of the Acoustic Monitoring Project of NOAA's Pacific Marine Environmental Lab, told The Atlantic for a short film about the sound in 2017.
Just what created the booming noise stumped scientists for years.
It wasn't until 2005, when NOAA embarked on an acoustical survey of the Antarctic off of South America, that scientists began to understand the bloop's origins.
Robert Dziak, from NOAA's Pacific Marine Environmental Lab, told Insider via email that by 2011 – after gathering all the data – the agency was able to definitively explain what the bloop was.
The official ruling: It was the sound of an icequake, created by the cracking of an ice shelf as it broke up from an Antarctic glacier.
The "sounds of ice breaking up and cracking is a dominant source of natural sound in the southern ocean," Dziak told Wired in 2012.
"Each year there are tens of thousands of what we call 'icequakes' created by the cracking and melting of sea ice and ice calving off glaciers into the ocean, and these signals are very similar in character to the bloop."
The icebergs that generated the bloop most likely were between Antarctica's Bransfield Straits and the Ross Sea, or Cape Adare, according to NOAA.
Icequakes occur when glaciers fracture in the ocean, cracking ice. The sudden cracking produces a loud pop or booming sound. With climate change, NOAA warns that icequakes are becoming more common.
Rising global temperatures melt glacial ice, creating water that can freeze again to cause an icequake.
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