Anchialine (from Greek meaning "near the sea") refers to coastal caves formed in limestone or volcanic rock that are flooded with seawater. They include the longest submerged caves on Earth. These caves are inhabited by a diverse array of previously unknown species from a number of new higher taxa. While some are primitive "living fossils", others are closely related to deep sea species. Most lack eyes and pigment, owing to their existence in the perpetual darkness of underwater caves.
Numerous caves and sinkholes in the Bahamas occasionally reach depths of 100 m, representing the lowest sea level from Pleistocene times. Such now submerged caves frequently contain stalactites and stalagmites, which only can form in air, thus confirming the long periods of lowered sea level these caves must have endured. Today, the groundwater on many island consists of a freshwater lens floating on underlying sea water.
Water-filled caves and cavities in the Bahamas are known as "Blue Holes" due to their predominant coloration. Such caves may occur in the interior of islands (inland blue holes) or in shallow waters on the banks (marine or ocean blue holes).
Three types of blue holes have been distinguished: (1) Cenotes are vertical shafts, frequently 50 to 150 m in diameter, that tend to bell out at depth and extend to 50 to 100 m depths. (2) Lens-based caves are laterally extensive systems formed at the contact zone between fresh and marine groundwaters. Lucayan Caverns, a lens-based cave on Grand Bahama, is the longest cave in the Bahamas at 14 km. (3) Fracture-guided caves are vertical, linear systems developed on major fractures running parallel to the steep edge of the carbonate bank. These fractures are believed to have formed as a result of slumping along the bank margin during periods of lower sea level. Such caves have passages 2 to 20 m wide, frequently reaching depths of 100 m or more.
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