17.3155°N, 87.5347°W.
Marine sinkhole 70 km off the coast of Belize within the Lighthouse Reef atoll. The hole is approximately 318 metres in diameter and 124 metres deep, formed by the collapse of a limestone cave system during glacial sea-level lowstands of the late Pleistocene (estimated formation 153,000 to 15,000 years ago, in stages). Stalactites and stalagmites recovered from depths below 90 metres confirm that the cave system was air-filled and growing speleothems before being submerged by post-glacial sea-level rise. Below 90 metres the water is anoxic and stratified; preserved within the stratification are the remains of organisms killed by hydrogen sulfide on the day they sank. The 2018 Aquatica expedition produced the first complete sonar map of the floor.
A cave system that was dry land 18,000 years ago is now a 124-metre hole in the Caribbean — and the bodies of fish that died on the day the seal broke are still on the floor.