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 are the Dragon Hole (in the South China Sea) and, in the Caribbean, the Great Blue Hole and Dean's Blue Hole.
Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater.
Description
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 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 . The diameter of the top entrance ranges typically from (Dean's Blue Hole) to (Great Blue Hole in Belize).
The overall largest blue hole (taking into account depth and width) is located 100 kilometers from the coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep.
Formation
Blue holes formed during past ice ages, when the sea level was as low as 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 in the case of the Sawmill Sink in the Bahamas.
The means in which blue holes are formed is through Karst processes. Karst processes need a specific type of topography. Rocks like limestone, gypsum and marble are soluble and so dissolution creates passages and cave systems underground. This paired with doline formation allows blue holes to be formed. Doline formation were once closed depressions formed by solution of superficial rock or subsidence collapse into an underground void.
Most blue holes are formed through these processes, however, some show no signs of passages or cave systems which are requirements of karst and doline processes. Therefore, these blue holes must have other origins like possibly vertical reef development.
Some blue holes however do not experience karst or doline processes during their formation. Some form through bedrock dissolution and collapse which in this case usually is controlled through tidal forcing, while others are controlled by carbonate dissolution, sea level fluctuations, and the presence of eogenetic carbonates.
Occurrence
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 deep spring basins formed by karst processes and located inland also are called blue holes, for example, Blue Hole in Castalia, Ohio.