The Florida Keys are not volcanic, not carved by glaciers, not deposited by rivers. They were built by living things. About 130,000 years ago, when sea levels stood higher than today, parallel lines of coral reef grew along the submerged Florida Platform. The northern and central Keys are exposed portions of this ancient reef, the Key Largo Limestone, while the lower islands from Big Pine Key to the Marquesas consist of Miami Limestone formed from oolites and bryozoan skeletons in the shallow sea behind the reef. Just offshore runs the Florida Reef, the third-largest barrier reef system in the world and the only one in North America, stretching from Fowey Rocks near Key Biscayne to the Marquesas Keys. The Keys were originally home to the Calusa and Tequesta peoples. When Juan Ponce de Leon charted them in 1513, he named them Los Martires, the Martyrs, because from a distance the low islands looked like suffering men.

For centuries the Keys were reachable only by water. Key West thrived on wrecking revenues from the treacherous reef, growing into the largest town in Florida, a hub of trade with Cuba and the Bahamas. Then Henry Flagler decided to extend his Florida East Coast Railway across 128 miles of open ocean. The Overseas Railway was an engineering marvel: dozens of trestles and bridges spanning channels and open water, built through three hurricanes in 1906, 1909, and 1910. The railway opened in the early 1910s, connecting Key West to the mainland for the first time. It ran for 23 years. On Labor Day 1935, the strongest hurricane ever to strike the United States at that time made landfall near Islamorada, killing more than 400 people including over 200 World War I veterans in highway construction camps. The storm destroyed the railway forever. Its bridges were converted into the Overseas Highway, U.S. Route 1, which still threads the chain of islands today.

The climate of the Keys is tropical savanna, closer to the Caribbean than to the rest of Florida. Freezing temperatures have never been recorded here. Royal poinciana trees bloom crimson in summer. Key deer, a miniature subspecies standing barely two feet tall, browse the pine forests of Big Pine Key under federal protection. American crocodiles patrol the mangrove shallows at the northern edge of their range. Mahogany, gumbo limbo, and thatch palms grow alongside temperate oaks and slash pines in a botanical crossroads found nowhere else in the continental United States. The waters surrounding the islands are protected as the Florida Keys National Marine Sanctuary, harboring more than 500 species of marine fish and over 45 species of stony coral. At night, the middle and lower Keys offer some of the last dark skies in South Florida, drawing astronomers to Scout Key's annual Winter Star Party.

Today the Overseas Highway stitches the Keys together in a single astonishing drive. U.S. Route 1 runs from Key Largo at Mile Marker 107 down to Key West at Mile Marker 0, crossing 42 bridges including the Seven Mile Bridge, one of the longest in the world when it was built, connecting Marathon to Little Duck Key. Most of the highway is just two lanes. There are no alternative routes. When a hurricane watch is issued, evacuation of the entire chain takes 12 to 24 hours. Monroe County has the highest per capita rate of fatal automobile accidents in Florida, a grim consequence of funneling hundreds of thousands of annual tourists onto a single narrow road through paradise. Yet the drive remains one of America's great road experiences, with turquoise water on both sides, pelicans wheeling overhead, and the remnants of Flagler's original railway bridges still standing alongside as fishing piers.

Key West sits just 90 miles from Havana, and the Cuban connection runs deep. After the Cuban Revolution, waves of emigres settled in the Keys, reinforcing cultural ties that had existed for centuries. Stories of Cuban rafters coming ashore are still not uncommon. The Keys have endured repeated devastation from tropical cyclones: the 1935 Labor Day hurricane, Hurricane Georges in 1998, and Hurricane Irma in 2017, which destroyed an estimated 25 percent of the houses on the islands and severely damaged another 65 percent. Homes are built on concrete stilts with breakaway walls. Residents live with the knowledge that no point in the Keys rises more than a few feet above sea level. Yet the population has grown to over 82,000, the tourism industry draws hundreds of thousands more each year, and the spirit of the Conch Republic endures: independent, salt-cured, and defiantly itself.