The Great Blue Hole is a massive marine sinkhole off the coast of Belize, near the center of Lighthouse Reef, a small atoll 70 km (43 mi) from the mainland and Belize City. The hole is circular in shape, 318 m (1,043 ft) across and 124 m (407 ft) deep. It was formed during several episodes of quaternary glaciation when sea levels were much lower.

Scuba divers and snorkelers have been cruising the surface waters for decades, but very few explorers have dared to venture deeper and explore what lies at the bottom. The first attempt to map the hole in its entirety was by Jacques Cousteau, who in 1971 he brought his ship, the Calypso, to the hole to chart its depths. His investigations confirmed its origin as a typical karst limestone formation, formed before rises in sea level.

In the winter of 2018, a crew from Aquatica Submarines ventured to the bottom of the Great Blue Hole in an attempt to map its interior. They made some unexpected discoveries.

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

Underneath that point there’s no oxygen, no life, only a graveyard of conch shells and hermit crabs that had fallen into the hole and suffocated. When the crew ventured further toward the bottom of the hole, around 120 meters deep, they found something they did not expect: small stalactites. The surprise gave scientists clues to the hole’s ancient past.

Stalactites can only form because water is dripping down stone. Therefore, scientists concluded this was a big, dry cave during a really prolific era on Earth, so there were probably lots of stuff living in it. They believe the cave formed during the last Ice Age, which ended about 14,000 years ago, when sea levels began to rise. At that point, 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 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. They also encountered two of the probable three people who have been lost in the Blue Hole. Yet, they decided they wouldn’t attempt any recovery. It’s very dark and peaceful down there, just they just let them stay at their eternal resting place.

Here’s footage of the crew’s descent:

Scientists predict the Great Blue Hole won’t be around forever to explore. As waterfalls of sand keep falling into it, it is slowly being filled up like an underwater hourglass. But as for now, we can still admire its beauty and study its many mysteries.