Great Blue Hole, Marine sinkhole at Lighthouse Reef, Belize

The Grand Trou Bleu is a marine sinkhole at Lighthouse Reef off the coast of Belize. The formation creates a nearly perfect circle in the ocean with a diameter of 300 meters (984 feet) and reaches depths of 123 meters (404 feet) below the water surface.

The geological structure formed during the last ice age when lower sea levels allowed limestone caves to develop. These cave systems became flooded around twelve thousand years ago as glaciers melted and sea levels rose, causing the cave roof to collapse and creating the opening visible today.

The site's name means "great blue hole" in English, and it became Belize's best-known dive spot after Jacques-Yves Cousteau's television film about it. Divers can still see the ledge at about 35–40 m on the south wall where the cave system opens, and the stalactites measured there reach up to 8 m long and nearly 2 m across.

Diving expeditions typically include one descent into the formation combined with two additional dives at nearby reef areas within Lighthouse Reef, departing from mainland Belize. The boat journey takes several hours and visitors require advanced diving certifications as currents and depth demand technical skills.

The submerged cave walls hold stalactites, some up to 8 m long and nearly 2 m thick, retrieved by Cousteau's Calypso divers from about 50 m depth. Many hang off-vertical by 5° in a consistent direction, evidence that the underlying plateau tilted after they formed.