Belize Blue Hole

The Great Blue Hole sits near the centre of Lighthouse Reef, an isolated atoll roughly 70 km off Belize City and the outermost of the country's three coral atolls. The hole is a flooded karst cave system that collapsed and drowned as sea levels rose after the last Ice Age: it is 318 m (1,043 ft) across, 124 m (407 ft) deep, and circular enough to be a landmark from orbit. Analysis of its submerged stalactites dates their formation to roughly 153,000, 66,000, 60,000, and 15,000 years ago, when the cave stood above sea level. Jacques Cousteau charted it with the Calypso in 1971 and declared it one of the world's top dive sites; a 2018 submersible expedition mapped a lifeless hydrogen-sulphide layer around 90 m and found two long-lost divers' remains at depth. The dive itself is a deep, dark descent—divers drop along a reef shelf to about 40 m to swim beneath house-sized stalactites with reef and nurse sharks circling in the blue; there is no coral inside the hole and little light, so it demands Advanced Open Water training. The genuine reward of a Lighthouse Reef trip is the atoll's surrounding wall diving—Half Moon Caye Wall, The Aquarium and Long Caye—where sheer drop-offs, 30 m-plus visibility and abundant Caribbean life far outshine the sinkhole. The atoll, the hole, and Half Moon Caye Natural Monument are all part of the Belize Barrier Reef Reserve System (UNESCO, inscribed 1996). Access is a long day-boat run from Ambergris Caye or Caye Caulker, or—better for the full atoll—a multi-day liveaboard.

• The Great Blue Hole is a circular karst sinkhole 318 m (1,043 ft) across and 124 m (407 ft) deep at the centre of Lighthouse Reef, formed when a limestone cave system collapsed and flooded as sea levels rose after the last Ice Age; submerged stalactites date its above-water formation to roughly 153,000, 66,000, 60,000, and 15,000 years ago.
• Jacques Cousteau brought the Calypso to the hole in 1971 to chart its depths and confirmed its origin as drowned karst limestone, declaring it one of the world's top dive sites; he documented internal ledges at about 21 m, 49 m, and 91 m. A 2018 submersible expedition later mapped a lifeless hydrogen-sulphide layer around 90 m and located the remains of two long-missing divers near the bottom.
• The Blue Hole dive is a deep, dark descent for experienced divers: groups drop along a reef shelf to about 40 m (131 ft) to pass beneath giant ancient stalactites, with reef and nurse sharks—and occasionally bull or hammerhead sharks—circling in the blue. There is no coral inside the hole and little marine life beyond the sharks, so the dive trades reef colour for geology and atmosphere and requires PADI Advanced Open Water certification.