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 t ...