Passage III

Blue Holes of the Bahamas

[1]

The Bahamas, a series of semitropical islands off the southeast coast of the United States, which are home to some of the most unusual geological formations in the world: underwater caves known as blue holes. [A] These vertical caves were formed over thousands of years, and their cold depths provide abundant clues to the islands’ past.

[2]

(32) During the formation process, tiny grains of calcium carbonate separated from the seawater. These grains built up, then compacted, forming the limestone that makes up the islands. [B] Over time, rainwater permeated the porous limestone but was trapped just above sea level, buoyed by the denser seawater below. Jostled back and forth by tides, the layer of slightly acidic, brackish water eroded limestone faster than either rainwater—or seawater—could alone. As the limestone eroded caves formed.

[3]

Over time periods in which the weather changed drastically, sea levels rose and fell by hundreds of feet. This allowed the cave-creating process to be a process that repeated at different depths hundreds of feet apart. The roofs of many caves collapsed, leaving the chambers beneath exposed. Some of these blue holes open to small contained caves others open to miles-long interconnected tunnels. (40)

[4]

The telltale sign of a blue hole is a circular patch of water striking darker than the water surrounding them. (The darker water indicates greater depth.) [C] Divers have found the remains of turtles and alligators. Now extinct on the islands, stalactites and stalagmites from a time when the caves were above sea level, and artifacts of early human inhabitants.

[5]

[D] Hundreds of blue holes can be sighted off the Bahamas. So far, most remain unexplored by divers, owing in part to the danger of cave diving. Often the only clue to the mysteries below is the tantalizing sight of dark blue water leading deep into the sea.