What the Great Blue Hole is
The Great Blue Hole is a nearly perfectly circular underwater sinkhole, about 300 metres across and 124 metres deep, formed thousands of years ago when sea level was much lower and water eroded a limestone cave that later collapsed. It sits at the centre of Lighthouse Reef atoll, about 70 km from the coast, within the Belize Barrier Reef Reserve System, a UNESCO World Heritage Site alongside the rest of the Belize Barrier Reef, the world's second largest after Australia's.
How to visit
Most trips leave from Ambergris Caye or Caye Caulker by speedboat, with a one- to two-hour ride to the atoll; diving into the Blue Hole itself is for advanced-certified divers, since you descend to around 40 metres to see the stalactite formations on the walls of the original cave, in deep, low-light water. Those without that certification, or who prefer to stay on the surface, usually combine the visit with snorkelling and diving elsewhere on the atoll, with far more visible marine life: nurse sharks, giant groupers and a wide variety of coral.
Before you book
As a deep, technical dive site, it's mandatory to go with a PADI-certified operator or equivalent, and some require a minimum number of prior dives before accepting a Blue Hole booking specifically. The full day, including the boat ride, usually lasts 10 to 12 hours, so it's worth booking early in the trip in case weather forces a reschedule.