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Scientists completed one of the most detailed explorations inside the Great Blue Hole.

There's a massive underwater sinkhole off the coast of Belize that extends 125 meters into the Earth's crust. It's called the Great Blue Hole. Scuba divers and snorkelers have been cruising the surface waters for decades, but few have dared to venture deeper and explore what lies beyond the blackness.

In the winter of 2018, a crew from Aquatica Submarines started their descent to the bottom of the Blue Hole. Their mission was to create a 3D map of the sinkhole's interior, but along the way, they came across some common and not-so-common sights.

As the crew started, they found the usual suspects: reef sharks, turtles, and giant corals. But as they pushed 90 meters, life started to vanish. The culprit was a thick layer of toxic hydrogen sulfide spanning the width of the entire sinkhole like a floating blanket.

Erika Bergman: Underneath that there's no oxygen, no life, and down there we found conchs and conch shells and hermit crabs that had fallen into the hole and suffocated, really.

Narrator: Past the conch graveyard and toward the bottom of the hole, around 120 meters deep, the team found something they did not expect: small stalactites. The surprise gave scientists clues to the hole's ancient past.

Bergman: Stalactites can only form because water is dripping down stone. And so we know that this was a big, dry cave, and it was during a really prolific era on Earth, so there were probably lots of stuff living in it.

Narrator: Scientists think the cave formed during the last Ice Age, which ended about 14,000 years ago. That's when sea levels began to rise, and the cave flooded and collapsed, leaving behind the Blue Hole we see today. Researchers think that other marine sinkholes, like Dragon Hole in South China Sea, and Dean's Blue Hole in the Bahamas probably formed the same way.

As the scientists continued down the hole, they found another clue to the past: a light buildup of silt on top of the conch graveyard.

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