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  • Narrator: 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.

  • Bergman: The silt itself on the bottom

  • is a pretty good record of all of the different hurricanes

  • and storm cycles and glaciations that have happened,

  • so we can see that right around the time

  • of the Mayan collapse, there were huge, huge storm cycles

  • followed by very significant droughts.

  • Narrator: As the team continued

  • to explore the bottom of the hole,

  • they found a 2-liter Coke bottle

  • and a lost GoPro containing some vacation photos.

  • But that wasn't all.

  • Bergman: We did encounter two of the probable

  • three people who have been lost in the Blue Hole,

  • so we found kind of the resting place of a couple folks,

  • and we just sort of very respectfully

  • let the Belize government know where we found them,

  • and everyone decided that we would just

  • not attempt any recovery.

  • It's very dark and peaceful down there,

  • just kind of let them stay.

  • Narrator: Scientists predict this hole

  • won't be around forever to explore.

  • Every day, waterfalls of sand fall into it,

  • slowly filling it up like an underwater hourglass.

  • But as for now, we can still admire its beauty

  • and study its many mysteries.

Narrator: There's a massive underwater sinkhole

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