The country containing the Great Blue Hole marine sinkhole is Belize.
The Great Blue Hole lies near the centre of Lighthouse Reef, a small atoll about 70 kilometres from Belize City and the mainland. It is a nearly circular marine sinkhole measuring roughly 318 metres across and about 124 metres deep. Its dark blue colour comes from the steep depth and contrast with the surrounding shallow reef waters.
The feature began as a limestone cave system when sea levels were lower during several stages of the Quaternary glaciation. As sea level rose, the cave flooded and its roof collapsed, producing the sinkhole seen today. Submerged stalactites provide evidence that parts of the cave formed above sea level.
Jacques Cousteau helped make the site famous after bringing the Calypso there in 1971. The hole is part of the Belize Barrier Reef Reserve System, a UNESCO World Heritage site. It is not in Mexico, Honduras or the Bahamas, although all are in the wider Caribbean region.