A perfect 318m navy-blue circle in a turquoise atoll, the most recognisable aerial image in diving. Divers descend the vertical…A perfect 318m navy-blue circle in a turquoise atoll, the most recognisable aerial image in diving. Divers descend the vertical wall to a 40m ledge hung with 12,000-year-old stalactites, then look down into black. Caribbean reef sharks and the odd bull shark patrol the rim. A limestone cave system that formed during the last ice age when sea level was 120m lower, then flooded and collapsed. Cousteau brought Calypso here in 1971 and blasted a channel through the reef to get in, an act that made it famous and would be unthinkable today. A 2018 submersible expedition found the bottom layered with anoxic sludge, conch shells and two bodies never recovered. Anchorage: Anchor inside Lighthouse Reef lagoon; the hole itself has mooring buoys, an overnight passage from Belize City. Hazards: No site-specific hazards noted, brief normally. Plus