Key derives from the Spanish word cayo, or little island. Keys are composed of sand and coral fragments built up just above sea level on a reef ﬂat. The Florida Keys curve southwest from Biscayne Bay to the Dry Tortugas, a distance of some 180 miles. The distance accessible by car is 106 miles from Key Largo to Key West, with the many genuine cayos, as well as a few ﬁll islands, linked by forty-two bridges. Numerous other keys in the chain, uninhabited and accessible only by boat, are protected by linked wildlife refuges and marine sanctuaries. The Keys rest on an ancient fossilized reef, but the upper keys are composed of porous limestone, while the lower keys have a layer of caprock, up to thirty feet deep, of ﬁnely grained calcium carbonate called Miami oolite. Rainfall drains quickly through the limestone but is retained in the compact, dense oolite, resulting in considerable differences in the natural environment. The bay—or Gulf—side of the Keys is called the backcountry. The Atlantic side is actually the Straits of Florida, where Hawk Channel runs out from the shore to the living reef. Beyond the reef is the Gulf Stream—“outfront”—and beyond that, the ocean. When Ponce de León sighted the Keys in 1513, he dubbed them Los Martires—the Martyrs—and sailed on.