Great Blue Hole

The Great Blue Hole is a large marine sinkhole in the centre of Lighthouse Reef Atoll, roughly 60 nautical miles off the coast of Belize City. The near-perfect circular shaft measures approximately 318 metres in diameter and reaches a depth of 124 metres, formed during glacial periods when sea levels were much lower and the original limestone cave system collapsed. Jacques-Yves Cousteau brought the site to international attention in 1971 aboard the Calypso, and it remains one of the most photographed dive sites on Earth. Divers descend along the outer rim wall and pass beneath enormous limestone stalactites and stalagmite formations between 30 and 40 metres, evidence of the cave's dry past. Below 50 metres a layer of hydrogen sulphide creates a hazy stratum where light fades and the water turns anoxic, so most operators cap the dive at 40 to 42 metres. Sightings of Caribbean reef sharks, blacktip sharks and the occasional bull or hammerhead are common in the upper bowl, while midnight parrotfish and groupers cruise the rim. Currents are typically gentle inside the hole. The site is a UNESCO World Heritage Site as part of the Belize Barrier Reef Reserve System and is best visited from April through June when seas are flattest. Advanced certification and a deep specialty are usually required.