The drive from Miami to the Keys is a slow descent into an unusual but breathtaking American ecosystem: On either side of you, for miles ahead, lies nothing but emerald waters. (On weekends, however, you will also see plenty of traffic.) Strung out across the Atlantic Ocean like loose strands of cultured pearls, more than 400 islands make up this 150-mile-long necklace.

Despite the usually calm landscape, these rocky islands can be treacherous, as tropical storms, hurricanes, and tornadoes are always possibilities. The exposed coast poses dangers to those on land as well as at sea.

When Spanish explorers Juan Ponce de León and Antonio de Herrera sailed amid these craggy, dangerous rocks in 1513, they and their men dubbed the string of islands "Los Martires" (The Martyrs) because they thought the rocks looked like men suffering in the surf. It wasn't until the early 1800s that rugged and ambitious pioneers, who amassed great wealth by salvaging cargo from ships sunk nearby, settled the larger islands (legend has it that these shipwrecks were sometimes caused by "wreckers," who removed navigational markers from the shallows to lure unwitting captains aground). At the height of the salvaging mania (in the 1830s), Key West boasted the highest per-capita income in the country.

However, wars, fires, hurricanes, mosquitoes, and the Depression took their toll on these resilient islands in the early part of the 20th century, causing wild swings between fortune and poverty. In 1938, the spectacular Overseas Highway (U.S. 1) was finally completed atop the ruins of Henry Flagler's railroad (which was destroyed by a hurricane in 1935, leaving only bits and pieces still found today), opening the region to tourists, who had never before been able to drive to this sea-bound destination. These days, the highway connects more than 30 of the populated islands in the Keys. The hundreds of small, undeveloped islands that surround these "mainline" keys are known locally as the "backcountry" and are home to dozens of exotic animals and plants. Therein lie some of the most renowned outdoor sporting opportunities, from bonefishing to spearfishing and -- at appropriate times of the year -- diving for lobsters and stone crabs. To get to the backcountry, you must take to the water -- a vital part of any trip to the Keys. Whether you fish, snorkel, dive, or cruise, include some time on a boat in your itinerary; otherwise, you haven't truly seen the Keys.

Of course, people go to the Keys for the peaceful waters and year-round warmth, but the sea and the teeming life beneath and around it are the main attractions here: Countless species of brilliantly colored fish can be found swimming above the ocean's floor, and you'll discover a stunning abundance of tropical and exotic plants, birds, and reptiles.

The warm, shallow waters (deeper and rougher on the eastern/Atlantic side of the Keys) nurture living coral that supports a complex, delicate ecosystem of plants and animals -- sponges, anemones, jellyfish, crabs, rays, sharks, turtles, snails, lobsters, and thousands of types of fish. This vibrant underwater habitat thrives on one of the only living tropical reefs on the entire North American continent. As a result, anglers, divers, snorkelers, and watersports enthusiasts of all kinds come to explore.

Heavy traffic has taken its toll on this fragile eco-scape, but conservation efforts are under way (traffic laws are strictly enforced on Deer Key, for example, due to deer crossings that have been contained, thanks to newly installed fences). In fact, environmental efforts in the Keys exceed those in many other high-traffic visitor destinations.

Although the atmosphere throughout the Keys is that of a laid-back beach town, don't expect many impressive beaches here, especially after the damaging effects of recent hurricane seasons. Nice beaches are mostly found in a few private resorts, though there are some small, sandy strips in John Pennekamp Coral Reef State Park, Bahia Honda State Park, and Key West. One great exception is Sombrero Beach, in Marathon, which is well maintained by Monroe County and is larger and considerably nicer than other beaches in the Keys. Sombrero Beach has a beachfront park, picnic facilities, a playground, and a protected cove for children.

The Keys are divided into three sections, both geographically and in this chapter. The Upper and Middle Keys are closest to the Florida mainland, so they are popular with weekend warriors who come by boat or car to fish or relax in such towns as Key Largo, Islamorada, and Marathon. Farther on, just beyond the impressive Seven-Mile Bridge (which actually measures 6 1/2 miles), are the Lower Keys, a small, unspoiled swath of islands teeming with wildlife. Here, in the protected regions of the Lower Keys, is where you're most likely to catch sight of the area's many endangered animals -- with patience, you may spot the rare eagle, egret, or Key deer. You should also keep an eye out for alligators, turtles, rabbits, and a huge variety of birds.

Key West, the most renowned -- and last -- island in the Lower Keys, is literally at the end of the road. The southernmost point in the continental United States (made famous by Ernest Hemingway), this tiny island is the most popular destination in the Florida Keys, overrun with cruise-ship passengers and day-trippers, as well as franchises and T-shirt shops. More than 1.6 million visitors pass through it each year. Still, this "Conch Republic" has a tightly knit community of permanent residents who cling fiercely to their live-and-let-live attitude -- an atmosphere that has made Key West famously popular with painters, writers, and free spirits, despite the recent influx of money-hungry developers who want to turn Key West into Palm Beach south.