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Blue holes, such as Watling's Blue Hole near San Salvador Island, are large marine sinkholes. They are typically located in areas where the ocean bedrock is made of soluble material (e.g., marble) because rock erosion is crucial to their formation. Because of the diverse marine life they contain, blue holes attract numerous divers and scientists. Currently the Taam Ja' Blue Hole in Chetumal Bay, Mexico, is the deepest blue hole on record, measured by Teresa Álvarez-Legorreta and colleagues to be at least 420 meters below sea level.