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A vast circular marine sinkhole at the heart of Lighthouse Reef Atoll, the Great Blue Hole is one of the natural wonders of the Caribbean. Seen from above, it is an almost perfect disc of midnight blue, about 300 metres across and 125 metres deep. It is ringed by pale turquoise shallows and a crescent of coral.

The hole formed as a limestone cave during the ice ages. It collapsed and flooded as sea levels rose. Below the surface, giant stalactites hang from ledges at around 40 metres. They are a record of the time when this was dry land. Reef sharks glide through the dim light. Experienced divers descend into the blue to see this geology for themselves. Snorkellers can explore the shallow reef edge, and flights over the atoll show the whole formation.

The site lies about 70 kilometres off the Belizean mainland. It is part of the Belize Barrier Reef Reserve System, a UNESCO World Heritage property. Most visitors arrive by boat on a long day trip from Belize City, or on a live-aboard. The crossing can be rough. In return you get open water, a quiet atoll and a sense of how remote the place is.