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.
Divers from around the globe visit this site for its importance in marine research and for the many documentaries filmed about the reef. Local captains and dive instructors often share stories about past expeditions and pass on their knowledge of the geological features to visitors.
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.
Reef sharks, large groupers, and Caribbean reef sharks inhabit the upper water layers while the underwater cave walls are lined with ancient stalagmites. These mineral formations grow at various angles and document several geological periods when the cave remained dry and dripstone accumulated.
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