Dean's Blue Hole near Clarence Town on Long Island, Bahamas, is the world's deepest known "blue hole" - a term given to any deep, water filled, vertical caves or sinkholes with an entrance below the water surface. 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 holes are so called for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. The intense blue color is created by high transparency of water and bright white carbonate sand. Blue light is the most enduring part of the spectrum; other parts of the spectrum - red, yellow, and green - are absorbed during their path through water, blue light manages to reach the white sand and return upon reflection.
Blue holes formed during past ice ages, when sea level was as much as 100-120 meters lower than at present. These holes were formed by deeper groundwater gradually dissolving the limestone until the ceiling of these voids collapsed. Later the sea level rose and filled the holes with water.
Dean's Blue Hole is a popular spot for diving ad snorkeling. At the 2012 freediving world cup in the Bahamas, New Zealand competitor William Trubridge set a world record when he dived to a depth of 121 meters without air in Dean's Blue Hole.