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 Great Blue Hole of Belize
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.
Dean's Blue Hole
Dean's Blue Hole near Clarence Town on Long Island, Bahamas, is the world's deepest known blue hole. While most blue holes and sinkholes reach a maximum depth of 110 meters, Dean's Blue Hole plunges to more than 200 meters, which makes it quite exceptional. At the surface, Dean's Blue Hole is roughly circular, with a diameter ranging from 25 to 35 meters. After descending 20 meters, the hole widens considerably into a cavern with a diameter of 100 meters.
Blue Hole of Dahab
This blue hole is located on the coast of the Red Sea, a few kilometers north of Dahab, Egypt. It is the second deepest blue hole at 130 meters. The Blue Hole has claimed the lives of many divers who tried to find the tunnel through the reef (known as "The Arch") connecting the Blue Hole and open water at about 52 meters depth.
The Blue Hole of Gozo
The Blue Hole is located on the west coast of the island of Gozo, in Maltese archipelago, in the Mediterranean Sea. It is a 15 meters deep and 10 meters wide hole in the bedrock of the cliff. Underwater, the Blue Hole is connected by a tunnel to the open sea.
Watling's Blue Hole
Some blue holes also occur on land, such as Watling's Blue Hole located on the Bahamian island of San Salvador. Despite sitting in the middle of a rainy tropical island, Watling’s blue hole has no fresh water indictaing that the hole must be connected to the ocean by an underground tunnel.
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