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:''For other uses and a list of blue holes, see Blue hole (disambiguation).'' 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. 〔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.〕 Well-known examples can be found in Belize, the Bahamas, Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea). ''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 392 meters (1,286 ft) is Pozzo del Merro in Italy. The 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). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「blue hole」の詳細全文を読む スポンサード リンク
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