Forty-three miles (70km) off the coast of Belize, the small atoll of Lighthouse Reef boasts a tantalizing phenomenon. The world's largest blue hole, the Great Blue Hole, is a submerged crater that draws divers the world over to explore its mysterious depths for themselves. Designated a UNESCO World Heritage Site in 1996, and a National Monument in 1999, diving Belize's Great Blue Hole is a geological wonder ripe for exploration.
How was it formed?
At 1,043 feet (318m) across and 407 feet (124m) deep, this huge sinkhole was created in separate stages during four different glacial periods where sea levels were far lower than they are currently. When the limestone plateau that now forms the seabed along the coast of Belize was above sea level, rainwater gradually eroded a huge cave that flooded and collapsed in on itself as sea levels began to rise. The walls of the Great Blue Hole display four distinct ledges at different depths which is why we know it was created in four stages. While there are many such examples of blue holes formed in this way, Belize's huge vertical hole is now the largest known example.
What is it like today?
Today, the Great Blue Hole offers divers the opportunity to discover unique marine life and an unusual diving experience along its steep walls. Unlike other inland sinkhole systems such as the cenotes dives in Mexico, this blue hole has virtually no horizontal tunnels or passageways eroded into its sides. However, it does display examples of huge stalagmites and stalactites, some as long as 40 feet (12m). These formations are further evidence that the blue hole's origins were as a limestone cave, and they become more complex with increasing depth.
Water temperatures within the Great Blue Hole tend to remain fairly constant at around 80F (27C), however, there is a noticeable thermocline at 90 to 100 feet (27 to 30m) where the water temperature drops to around 72F (22C). Below the thermocline, visibility improves, although it does become gloomy as less sunlight penetrates to these depths.
What can I expect to see?
The stark nature of the blue hole's formation makes it fairly inhospitable to the majority of Caribbean reef species. A small number of fish have made their home in the overhangs and ledges of the hole's steep sides, however, a lack of water flow and minimal nutrients keeps fish numbers to a minimum. Many of the visiting day boats chum the water, and Caribbean reef sharks, bull sharks, and even hammerheads are regular visitors to the shallower reaches of the site.
The true beauty of the Great Blue Hole isn't apparent until below 100 feet (30m), where the monstrous stalactites and stalagmites start to appear. These formations, some as big as cars, add a surreal and prehistoric feel to the dive, their telltale striations giving explorers a clue to their past. Many of these columns sit at a consistently off-vertical angle, suggesting geological shifts played a secondary part in their formation.
The lip of the Great Blue Hole sits at around 25 to 35 feet (7 to 10m) underwater and is surrounded by a ring of dispersed coral heads. Divers and snorkelers can discover tiny critter and macro life such as corkscrew anemones, Pederson’s cleaner shrimp, slender filefish, and various gobies and blennies. There are also plenty of butterflyfish and schooling species such as jack in the shallower waters.
How do I dive the Great Blue Hole?
Only accessible by boat, day trips are available from coastal ports in Belize, however, to truly explore this geological phenomenon, a liveaboard is the best option. Embarking and disembarking from Belize City, liveaboard itineraries tend to last for 7 nights and take in the best sites of Lighthouse Atoll, Half-moon Cay, and Turneffe Reef.
From the US, there are regular direct flights to Belize's Philip S.W. Goldson International Airport (BZE). The Great Blue Hole can be dived year-round, however, April to June offers the best visibility and a better chance of spotting passing whale sharks.
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