from Yahoo Answer
It takes bravery and special training to venture into the hydrogen sulfide atmosphere of the Bahama caves known as inland blue holes. Offshore flooded caves, so-called ocean blue holes, are extensions of the sea, subject to the same heavy tides and host to many of the same species found in the surrounding waters. Inland blue holes, however, are unlike any other environment on Earth, thanks largely to their geology and water chemistry. In these flooded caves, such as Stargate on Andros Island, the reduced tidal flow results in a sharp stratification of water chemistry. A thin lens of fresh water—supplied by rainfall—lies atop a denser layer of salt water. The freshwater lens acts as a lid, isolating the salt water from atmospheric oxygen and inhibiting bacteria from causing organic matter to decay. Bacteria in the zone just below the fresh water survive by exploiting sulfate (one of the salts in the water), generating hydrogen sulfide as a by-product. Known on land as swamp or sewer gas, hydrogen sulfide in higher doses can cause delirium and death.
http://www.neatorama.com/2010/07/18/cave…
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; where other parts of the spectrum - red, yellow, and finally green - are absorbed during their path through water, blue light manages to reach the white sand and return back upon reflection.
The deepest blue hole in the world—at 202 metres (663 ft)—is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. 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 subjected to 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 both 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.
http://en.wikipedia.org/wiki/Blue_hole
Blue holes can run extremely deep underground, with one Bahamian blue hole exceeding 600 feet (180 meters) below sea level, and contain a series of mazelike passageways going miles in many directions. These cave systems can transition from giant rooms to tiny holes that divers must remove all of their gear in order to squeeze through. To add to the challenge, currents reverse in the ocean caves, making timing of dives critical. In the Bahamas , the inland underground systems serve as aquifers that contain the rain water that percolates down through the porous limestone. In these geologic reservoirs, a thin lens of the lighter freshwater floats above the denser salt water. In addition to its critical role as a source of potable water, this underground world is home to dozens of new species, including a new class of crustaceans. The Bahamian archipelago spans almost 1,000 miles (1,609.3 kilometers), allowing the team to test theories of human and animal migration, and to reconstruct regional climate back hundreds of thousands of years. These reconstructions can help us understand the extent and rates of global sea level rise as well as occurrences of abrupt climate change, both important issues in the face of global warming.
http://www.nationalgeographic.com/field/…
Blue holes in the Bahamas are usually classified as one of several types: (1) Sinkholes, which are the large round blue holes usually seen from the air and are the type of formation where the Blue Holes get their name. (2) Fault line or fracture, which arevery deep caves are usually associated with localized faulting. The systems generally run parallel to a deep-water, offshore canyons such as the Tongue of the Ocean east of Andros Island, or the Exuma Sound, east of the Exuma Cays. (3) Lens based blue holes are usually the longest of the blue holes and are often highly decorated with speleothems. They are the result of a well-defined fresh water lens, actively dissolving a vast system of passages into the limestone. These are also known as flooded flank margin caves.
http://www.neatorama.com/2010/07/18/cave…
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; where other parts of the spectrum - red, yellow, and finally green - are absorbed during their path through water, blue light manages to reach the white sand and return back upon reflection.
The deepest blue hole in the world—at 202 metres (663 ft)—is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. 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 subjected to 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 both 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.
http://en.wikipedia.org/wiki/Blue_hole
Blue holes can run extremely deep underground, with one Bahamian blue hole exceeding 600 feet (180 meters) below sea level, and contain a series of mazelike passageways going miles in many directions. These cave systems can transition from giant rooms to tiny holes that divers must remove all of their gear in order to squeeze through. To add to the challenge, currents reverse in the ocean caves, making timing of dives critical. In the Bahamas , the inland underground systems serve as aquifers that contain the rain water that percolates down through the porous limestone. In these geologic reservoirs, a thin lens of the lighter freshwater floats above the denser salt water. In addition to its critical role as a source of potable water, this underground world is home to dozens of new species, including a new class of crustaceans. The Bahamian archipelago spans almost 1,000 miles (1,609.3 kilometers), allowing the team to test theories of human and animal migration, and to reconstruct regional climate back hundreds of thousands of years. These reconstructions can help us understand the extent and rates of global sea level rise as well as occurrences of abrupt climate change, both important issues in the face of global warming.
http://www.nationalgeographic.com/field/…
Blue holes in the Bahamas are usually classified as one of several types: (1) Sinkholes, which are the large round blue holes usually seen from the air and are the type of formation where the Blue Holes get their name. (2) Fault line or fracture, which arevery deep caves are usually associated with localized faulting. The systems generally run parallel to a deep-water, offshore canyons such as the Tongue of the Ocean east of Andros Island, or the Exuma Sound, east of the Exuma Cays. (3) Lens based blue holes are usually the longest of the blue holes and are often highly decorated with speleothems. They are the result of a well-defined fresh water lens, actively dissolving a vast system of passages into the limestone. These are also known as flooded flank margin caves.
Source(s):
Each of these different types of caves could be associated with or connected to each other. For example, Angelfish Blue Hole in the Exuma Cays is predominantly a lens-based cave that connects through several passages to a major local fault line cave system. After their initial creation, fault line caves continue to enlarge by the same chemical processes as lens-based caves. Because of its subsurface entrance this cave is also considered a marine or ocean blue hole. Another example is Bottomly's Blue Hole and Mystery Cave on Stocking Island, Exuma. Bottomly's is an inland (Anchialine) blue hole, that is connected by a labyrinth of passages to Mystery cave, which is a very tidally effected marine blue hole.
http://www.bahamascaves.com/blueholes.ht…
http://www.bahamascaves.com/blueholes.ht…
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