The Blue Hole, Dahab

The Blue Hole is a roughly circular sinkhole 100 metres deep, separated from open water by a shallow saddle of reef known as the Bells. For snorkellers it is a straightforward and spectacular swim over the rim, with vertical coral walls dropping into deep blue.

For divers it has a fearsome reputation, entirely due to the Arch — a 26-metre tunnel at 55 metres that has killed a significant number of unqualified divers attempting it. The recreational route over the saddle is safe and beautiful.

A drowned karst feature formed during the last ice age when sea levels were far lower. It is also one of the world's premier freediving sites and hosts regular competitions.