You are floating in a sunlit Caribbean reef when the seafloor simply vanishes, dropping over 400 feet straight down into a perfectly circular pitch-black abyss. It looks like a portal to another dimension hiding in shallow tropical water.
This episode descends through Belize's Great Blue Hole, a marine sinkhole that is also an ancient geological record. We trace its journey from a dry Ice Age cave to a flooded chasm, and explore the toxic dead zone at its lifeless bottom.
How rainwater carved a karst limestone cave over tens of thousands of years when sea levels were far lower
The ledges at 69, 161, and 299 feet that record at least four distinct stages of ancient sea-level rise
Cousteau's 1971 expedition and the 40-foot stalactites that proved the cavern was once dry, including ones tilted 5 degrees by shifting tectonic plates
The 300-foot hydrogen sulfide layer that creates a strict halocline and an anoxic, lifeless dead zone below
The 2018 submarine sonar mapping mission that discovered two missing divers, left undisturbed as a burial at sea
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