The aquifer beneath south Florida is called the Biscayne Aquifer and is one of the most permeable in the world. Rainwater entering it is rapidly discharged into surrounding coastal waters. The Florida Keys consist, for the most part, of porous limestone, remnants of an ancient coral reef. However, on the large islands of the Lower Keys—Key West, Sugarloaf, Cudjoe, and Big Pine—a thick caprock of ﬁne-grained calcium carbonate retains water in thin lenses that actually ﬂoat on top of the heavier underlying saltwater. The size and extent of the lenses vary depending on the elevation of the key beyond the reach of tides, the season, and, of course, the amount of rainfall. Lenses are of maximum size and freshness during the late-summer wet season and are closer, at that time, to the surface. A freshwater lens—varieties of which occur in many coastal areas of the world—is highly beneﬁcial for wildlife, but any large-scale pumping for human use can quickly exhaust it, and saline water could be rapidly drawn into the aquifer. The Keys rely on water piped from wellﬁelds in Florida City, 130 miles from Key West. The original pipeline was built by the U.S. Navy in 1940.