At a Glance

In a blog post, the pilot of the sub on the mission describes giant stalactites encrusted with marine creatures.At 300 feet, they found a layer of highly corrosive hydrogen sulfide.There's no oxygen at the bottom of the sinkhole, which may explain all the dead conchs.

Giant stalactites covered in marine growth, a conch graveyard and a layer of corrosive hydrogen sulfide are among the surprising finds made by a team of scientists that journeyed to the bottom of the world's largest underwater sinkhole.

The expedition to the Great Blue Hole off the coast of Belize had two goals: create a 3D sonar map of the sinkhole and share images of part of a with the world.

The team included Richard Branson, billionaire founder of Virgin; Fabien Cousteau, grandson of famous explorer Jacques Cousteau; and Erika Bergman, oceanographer and chief pilot of the three-person sub used in the mission.

Last month, Branson wrote in a journal that the team had during the trip that took place in November and December.

In a more recent blog post about the expedition, Bergman in the sinkhole that is 407 feet deep and more than 1,000 feet across.

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She said one area they studied in detail was stalactite caverns that are 130 to 160 feet deep. These mineral icicles formed when the Great Blue Hole was a dry cave and dripping water left calcium salt deposits.

"These enormous stalactites have now been entirely encrusted in marine growth," Bergman wrote.

The limestone and calcium carbonate walls of the sinkhole are terraced in spots, evidence of erosion that happened after sea level rose. Between the terraces, Bergman said, the walls are smooth and erosion-free because sea level rose rapidly then.

About 300 feet down sits a thick layer of corrosive hydrogen sulfide gas. Bergman said the expedition's equipment was examined rigorously for signs of corrosion.

"There’s not a drop of oxygen below the H2S layer," Bergman wrote.

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The team has named an area at the north entrance of the hole, about 50 feet below that layer, the "Conch Graveyard."

It appears, Bergman said, that thousands of conch fall over the rim of the hole and settle on the bottom.

"We can see each conch with little tracks back up the hill trying to escape, then a slide mark where it slid back down after presumably being asphyxiated in the anoxic environment," she wrote.