A blue hole is an underwater sinkhole formed by the erosion of carbonate rocks and appears as a dark blue circle of water in the ocean. Blue holes are typically located in low-lying coastal regions, which were once above the sea level many thousand years ago. Intense karst activity – the process of dissolution of soluble rocks such as limestone, dolomite, and gypsum by rain water or streams – created large vertical caves. When the sea level rose due to melting of glaciers, some of these holes became submerged. Owing to their depth, blue holes appear darkish blue because of greater absorption of sunlight which increases with increase in depth. This creates a dramatic contrast with the lighter blue of the shallows around them and forms a natural outline that can be easily seen from the surface.
The most famous blue hole is located off the coast of Belize, a small country on the eastern coast of Central America. It lies near the center of Lighthouse Reef, a small atoll 70 km from the mainland and Belize City. The Great Blue Hole is over 300 meters across and 124 meters deep, and was formed during several episodes of quaternary glaciation between 150,000 to 15,000 years ago. It’s a popular spot among recreational scuba divers.