A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock (limestone or coral reef). Blue holes typically contain tidally-influenced water of fresh, marine, or mixed chemistry. They extend below sea level for most of their depth and may provide access to submerged cave passages. Well-known examples can be found in South China Sea (Dragon Hole), Belize, the Bahamas, Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea).
Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria. The deep blue color is caused by the 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 finally green—are absorbed during their path through water, but blue light manages to reach the white sand and return upon reflection.
The deepest blue hole in the world at 300.89 meters (987 feet) deep is in the South China Sea and is named the Dragon Hole, or Longdong. The second deepest blue hole in the world with underwater entrance at 202 metres (663 ft) is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. Other blue holes are about half that depth at around 100–120 metres (330–390 ft). The diameter of the top entrance ranges typically from 25–35 metres (82–115 ft) (Dean's Blue Hole) to 300 metres (980 ft) (Great Blue Hole in Belize).
Blue holes formed during past ice ages, when sea level was as much as 100–120 metres (330–390 ft) lower than at present. At those times, these formations were targets of the same erosion from rain and chemical weathering common in all limestone-rich terrains; this ended once they were submerged at the end of the ice age.
Most blue holes contain freshwater and saltwater. The halocline is the point in these blue holes where the freshwater meets the saltwater and where a corrosive reaction takes place that eats away at the rock. Over time this can create side passages, or horizontal "arms", that extend from the vertical cave. These side passages can be quite long; e.g., over 600 metres (2,000 ft) in the case of the Sawmill Sink in the Bahamas.
Blue holes are typically found on shallow carbonate platforms, exemplified by the Bahama Banks, as well as on and around the Yucatán Peninsula, such as at the Great Blue Hole at Lighthouse Reef Atoll, Belize.
Many different fossils have been discovered that indicate the type of life forms that existed in blue holes. Other life forms such as marine life and marine fossils have also been noticed; Crocodile and tortoise fossils, for instance, have been found in blue holes.
- Mylroie, J. E., Carew, J. L., and Moore, A. I., (1995), Blue Holes: Definition and Genesis: Carbonates and Evaporites, v. 10, no. 2, p. 225.
- Keen, Cathy (December 3, 2007). "Fossils excavated from Bahamian blue hole may give clues of early life.". University of Florida. Retrieved April 24, 2016.
- Schwabe, Stephanie; Carew, James L (2006). "BLUE HOLES: AN INAPPROPRIATE MONIKER FOR SCIENTIFIC DISCUSSION OF WATER-FILLED CAVES IN THE BAHAMAS" (PDF). The 12th Symposium on the Geology of the Bahamas and other Carbonate Regions. Retrieved 1 February 2017.