When seen from above, the Great Blue Hole looks like the pupil of an eye. Seen from within, this Unesco World Heritage–listed ocean sinkhole is a visual treat for divers. Ringed by fringing reef, and approximately 400m in diameter, the Great Blue Hole drops away to around 145m. About 40m down are the formations that lure divers from around the world: marine stalactites up to 15m in length. Marine life is noticeable only in its absence – you might not see a single fish – but ...