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 m, Dean's Blue Hole plunges to more than 200 m, which makes it quite exceptional. At the surface, Dean's Blue Hole is roughly circular, with a diameter ranging from 25 to 35 m. After descending 20 m, the hole widens considerably into a cavern with a diameter of 100 m.
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 colour 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.
