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.