The Great Blue Hole - Belize. The 'Great Blue Hole' is 124m deep and part of the famous Lighthouse Reef, 70km from mainland Belize. The father of SCUBA, Jacques Costeau, called it one of the 'top 10 places to dive in the world' – and it's now a UNESCO World Heritage Site. It's not the deepest though. It sinks to just over 200m – out of reach for all but the most extreme divers.