Blue Holes of the Bahamas

Blue Holes are named for the dramatic contrast between the lighter shades of the surrounding shallow water and the dark, deep blue holes where these natural formation plunge into the abyss. The deepest, Dean's Blue Hole, located in bay west of Clarence Town on Long Island, Bahamas, reaches 202 meters (663ft). Many others are located in the same region (the Bahamas) as well as Belize, Guam, Australia and the Red Sea. 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.

Images from neatoramam & Un Monde Ailleurs

And It's not just incredible color palettes with unique contrasts and quintessential deep sea greens and blues that sceintist are discovering:

Swimming holes they are not. The inland caves on five islands sport freshwater caps covering heavier saltwater layers, sometimes filled with clouds of poisonous hydrogen sulfide released by salt-eating microbes, acting to preserve whatever falls within. Others contain whirlpools powered by the tides. - USA Today

Environmental Graffiti

Those who do venture into the watery veins of the Earth discover whole ballrooms full of tightly packed stalactites, prehistoric human remains, and fossils of now extinct crocodiles and tortoises. These caves are, quite literally, another world. - National Geographic

USA Today: In submerged caves such as Stargate on Andros island in the Bahamas, the expedition team reports:
• Specialized "chemosynthetic" bacteria that live without oxygen and feast on chemical reactions possible only in the caves.
• Stalactite curtains, or "speleothems," that contain a record of past sea level and climate conditions locked in their structures.
• Fossils of Lucayan tribe members who lived on the islands until the 1500s.

Images unless otherwise noted are from National Geographic and were taken by Wes C. Skiles. Tragically, Wes C. Skiles died in July of 2010 while filming underwater in Florida. Bahamas Blue Holes was he last story. Prints of the photos are available here. Text adapted from wikipedia.