Cathedrals of calcite The Bahamas, a country of more than 3,000 islands, cays and islets strewn like puzzle pieces across the Atlantic Ocean, harbours an underwater alien universe that few people have probed. Called “blue holes” for their deep navy hue, these flooded caves are spectacular calcite cathedrals of stalactites, stalagmites, draperies and fine, cylindrical deposits called straws. There is estimated to be more than 1,000 such caves in the Bahamas, only 20% of which have been explored. Pictured here is the Cascade Room, a passageway 24m underground that leads into Dan's Cave, a blue hole on Abaco Island in the northern Bahamas. (Wes C Skiles/National Geographic Stock) Deep plunges Located in a protected cove on Long Island, Dean's Blue Hole -- Earth's deepest known underwater cave -- plunges 183m into darkness. Offshore flooded caves are subject to the same tides as the ocean and are home to many of the same species found in the surrounding waters. (Wes C Skiles/National Geographic Stock) Underwater forests Following a guideline and taking care not to touch anything (a single, misplaced fin kick by a diver can shatter mineral formations tens of thousands of years old), an underwater explorer threads her way through a stalagmite forest in Dan's Cave on Abaco Island. From a diver's perspective these caves are on a level with Everest or K2, requiring highly specialised training, equipment and experience, and the ability to work under tremendous time pressure. In these labyrinths, separation from the guideline can be fatal. (Wes C Skiles/National Geographic Stock) Turbulent waters Like a giant bathtub drain, a vortex in Chimney Blue Hole off the island of Grand Bahama sucks down millions of litres of sea water when the tide comes in. In the background of the photo, a diver sets up equipment to measure the whirlpool's flow rate. (Wes C Skiles/National Geographic Stock) Flooded voids Here, divers surface Sawmill Sink, an inland blue hole on Abaco Island. Inland blue holes are unlike any other environment on Earth. In these flooded voids, the reduced tidal flow results in a sharp stratification of water: a thin lens of fresh water — supplied by rainfall — lies atop a denser layer of salt water. (Wes C Skiles/National Geographic Stock) Preserved fossils A diver lifts a more than 3,000-year-old Cuban crocodile skull -- an animal no longer found in the Bahamas -- from sediment in Sawmill Sink. Virtually oxygen free, blue holes preserve bones intact; and scientists have found the preserved bones of crocodiles, tortoises, bats, owls, beetles and other species that thrived in the Bahamas long before people. In these dark waters, soft tissue still clings to thousand-year-old tortoise shells; leaves retain their structure and pigments; insect wings remain iridescent. (Wes C Skiles/National Geographic Stock) New species In lightless blue holes, animals like this inch-long Agostocaris cave shrimp lack almost all pigmentation; only part of the shrimp's digestive system has colour. In recent decades, scientists have discovered an abundance of previously unknown organisms in these and other flooded caves around the world -- more than 300 new species, 75 new genera, nine new families, three new orders as well as a new class of blind crustaceans called Remipedia, first documented in 1981 in the Bahamas. Most cave-adapted species are crustaceans, many of which are "living fossils" -- live species closely resembling those preserved in fossils. (Wes C Skiles/National Geographic Stock)