A Submarine Went to the Bottom of the Great Blue Hole. It Used to Be a Cave on Dry Land

Ask what carved the Great Blue Hole, and most people will say the sea did. A dark circle about a thousand feet across, sunk into a reef off Belize. Except its walls are hung with stalactites, and stalactites only grow in open air. So this hole in the ocean was once a cave on dry land. When was that? And what did a submarine find on its floor? In this documentary we go from the boat to the base of the Great Blue Hole on Lighthouse Reef, about 407 feet down. The ice ages that dropped the sea around 400 feet and turned the atoll into a limestone island; the stalactite caverns at 130 to 160 feet; Jacques Cousteau's visit in the early 1970s and geologist Robert Dill's ledges that record old sea levels; the divers who reached the floor in 1995 and 1997; the 2018 Aquatica submarine dives with pilot Erika Bergman, Richard Branson and Fabien Cousteau; the hydrogen sulfide layer near 300 feet with no oxygen below it; the Conch Graveyard; and the two very different accounts of plastic on the bottom. Then the 2022 sediment core: about 98 feet of mud showing the hole was a partly flooded land sinkhole, a cenote, until the sea took it roughly 7,000 years ago, plus 574 storm layers over 5,700 years and the droughts of the Maya collapse. Where sources disagree (Cousteau's year, the exact depth, the depth of the gas layer, the plastic), we say so. And we check the scarier claims about the hole against the expedition record. 0:00 Hook 0:24 Rain in a Sunken Cave 2:13 The Postcard From Above 3:52 An Island Made of Old Reef 6:21 Two Small Subs and a Geologist 9:06 The Lid at Three Hundred Feet 12:16 The Conch Graveyard 14:14 Bottles, or a Few Scraps 16:18 A Platform Over the Hole 18:52 A Diary of Storms 21:17 Surface to Base 22:50 What Nobody Has Answered 24:13 Outro Subscribe to Depthmark to see what lies beneath — measured. #GreatBlueHole #Depthmark #Belize #EarthScience #Documentary