The first indication that things might be just a little bit different here are the bright yellow signs emblazoned with the likeness of a black alligator.

It takes a second to sink in. They are alligator crossing signs. Alligators cross the road here. If alligators cross the road here, then alligators live here, probably right there in those mangrove swamps trying to overtake the highway.Suddenly the two-lane blacktop known as U.S. 1 seems very narrow indeed, and a little more pressure on the accelerator appropriate.

Welcome to the Florida Keys, an ant trail of coral outcroppings that dribbles its way 126 miles southward from the tip of Florida's chin. It is a place of ticky-tacky motels, swanky resorts, an alarming amount of pink flamingo yard art and an enticing number of wooden lobster traps stacked roadside.

A little bit decadent and a whole lot dank, the Keys are a place of seas so turquoise they appear unreal and a pace so languid that the just-arrived may feel they've been superimposed into a movie that's running in slow motion.

This is a place to drop in and drop out. People do. So much so that the post office in Islamorada has two slots, one for Islamorada, the other for "The World."

What this other-worldly place offers is as simple as it is hedonistic. From Key Largo to Key West, the focus is on the water. Get up in the morning. Have a traditional Keys breakfast of fried grouper and grits. Dive, snorkel, fish, swim or sail. Have a nice dinner. Sleep. Get up and do it all over again.

Key Largo is known for diving, Islamorada for deep-sea fishing, Marathon for ecotourism and Key West for partying.

Before setting out to conquer the Keys, though, a person should know three things. First, this is not a place of natural sandy beaches. Those few to be found are usually man-made. Second, many of the better diving and snorkeling sites are well offshore and accessible only by boat.

And third, to function well in the Keys, a visitor needs to learn the local language. It's called "mile marker" after those small green signs with white numbers posted along U.S. 1. Their vocabulary begins just south of Florida City on the mainland with Mile Marker 127 and ends with Mile Marker 0 in Key West.

Ask a local where to find, say, Island Silver & Spice, and the answer will come back: "Just past Mile Marker 82 in Islamorada." Actually, it makes sense, and it's a lot easier to learn than French.

Upper Keys

Key Largo not only is home to the African Queen, the tattered steamer from the Humphrey Bogart/Katharine Hepburn movie of the same name, it's also home to Molasses Reef, one of the Keys' prime diving spots and the only living coral reef in the continental United States.

The African Queen, newly restored to look old, has taken up residence at the local Holiday Inn, making an interesting juxtaposition with the Holiday Inn's new gambling boat, the Pair-A-Dice.

The reef, however, is genuinely old, the world's oldest, in fact, and home to 650 types of marine life and 40 varieties of coral. Diving and snorkeling can be arranged through John Pennekamp Coral Reef State Park or area dive shops.

Key Largo Dry Rocks, where a 9-foot bronze statue of Christ, his arms uplifted, is submerged in 25 feet of water, is another popular dive site in the Key Largo National Marine Sanctuary. The nearby wreck of the submarine Benwood is scattered over 300 feet of the ocean floor and makes a cozy home for moray eels and lobsters. Experienced divers head for the twin U.S. Coast Guard cutters Bibb and Duane just outside the reef.

The less adventurous may snorkel from shore at Pennekamp, the world's first underwater park, in several marked swimming areas, where 14 cannons and an anchor are visible.

For an up close but not too personal look at Molasses Reef, book a ride on the San Jose, Pennekamp's glass-bottomed boat. On the way out, a guide talks about the area's delicate ecology and efforts to preserve it. Once over the reef, the San Jose slows to an idle, the guide descends a ladder into the boat's glass hull and the show begins.

A purple sea fan does a Sally Rand dance in the current to an audience of yellowtail snappers and parrotfish. A barracuda zips away when we linger a little too long for comfort. A ray, like some underwater flying saucer, undulates up from the bottom and disappears to the right.

At the Key Largo Undersea Park, humans provide entertainment for nurse sharks, giant angelfish and lobsters at the Jules Undersea Lodge, an underwater hotel complete with VCR and mer-chef. Dive in, dive out. Claustrophobics need not apply.

Ecology-themed boat tours are another popular way to take in the Upper Keys. Tours booked through Caribbean Water Sports traverse Florida Bay to mangrove islands that serve as nurseries for black-tip, bonnet-head and nurse sharks, all easy to spot in the clear, shallow waters.

The tours also allow a bird's-eye view of pelicans as they fish, and the once near-extinct white crowned pigeons, members of the party-hearty crowd. It seems the pigeons have a taste for the intoxicating red berries produced by poisonous trees, and they indulge that taste to the point of getting so drunk they fall out of the trees. Or are they just imitating the tourists back at the beach?

Middle Keys

The Dolphin Research Center on Grassy Key is home to 14 bottlenose dolphins, all of whom were either born here or brought to the center for rehabilitation because of medical problems.

This was Flipper's home and the filming site for the movie of the same name. No joke, Flipper really is dead. But his daughter, Tursi, and grandson, Talon, are current residents.

The center hosts tours, as well as highly-structured dolphin encounters in which participants spend an hour-and-a-half learning about these playful, intelligent mammals and 20 minutes in the water with two dolphins and a trainer.

Delphi the dolphin learned to imitate a trainer's giggle and taught it to the other dolphins, Santini is an accomplished acrobat, and Theresa proves to be quite her own woman, giving her trainer a raspberry when she doesn't like a command.

The center is part of the Southeast Stranding Network, which aids stranded dolphins from Key Largo to Key West. It is also home to a highly successful program in which the dolphins, under supervision of trainers and members of the medical community, interact with children who are physically challenged, autistic or have Down's syndrome.

Theater of the Sea at Islamorada is strictly commercial, with continuous shows starring rays, turtles, sharks, sea lions and dolphins. They too offer a structured dolphin swim program.

The San Pedro Underwater Archaeological Preserve, about a mile south of Indian Key, is site of a 1733 Spanish shipwreck in 18 feet of water. Ideal for snorkelers and novice divers, the site features cannon replicas, trails, coral formations and schools of tropical fish. Experienced divers enjoy the Eagle, a freighter, just off Islamorada.

Marathon could be just about any small city, complete with fast-food franchises and strip shopping centers, until you get to its southern tip where Seven Mile Bridge soars out over the water into the horizon.

That patch of land about two miles out is Pigeon Key National Historic District. To get there, park where the old and new bridges meet, and hop aboard the "conch cruiser," a rickety '75 Mercedes minibus that shudders, rattles and clanks its way over the old bridge to the five-acre island.

The smattering of white wooden buildings on Pigeon Key were constructed in 1908-1909 to house workers building the Florida East Coast Railway, an ambitious undertaking by Henry Flagler that linked Homestead, Fla., to Key West via rail. The railroad was finished in 1912 and destroyed by the Labor Day hurricane of 1935. It was replaced with the Overseas Highway in 1938.

Those side rails on the bridge are remnants of the old Flagler railroad. Not much else is left except the stories.

This is the Keys as they were before the sleek resorts, even before the mom-and-pop motels. It's very quiet here with the wind whirring in the trees and birds carrying on discreet conversation. The air smells of pine needles and the sea. It is 10 minutes and a million miles away from the McDonald's in Marathon.

Lower Keys

The Lower Keys tend to appeal most to hearty folk intent on the serious pursuits of fishing and diving.

Looe Key National Marine Sanctuary, named for a British frigate that ran aground there in a 1744 storm, is their kind of place. Its varied and spectacular coral reefs are accessible by boat just offshore from Big Pine, Ramrod and Summerland keys.

Bahia Honda State Recreation Area boasts the best beach in the Keys, a real, honest-to-goodness white sand shore with gentle surf perfect for swimming. This pretty park also has nicely secluded campsites, sheltered picnic tables, a restaurant, a boat ramp and a dive shop.

Tiny Key deer, weighing 45 to 75 pounds and standing about 30 inches tall, are protected at the Key Deer National Wildlife Refuge on Big Pine Key. They tend to be elusive critters, making an appearance suddenly in front of headlights when it's too late to stop. The deer, a subspecies of Virginia white-tailed deer, sometimes forage at the edge of the roadside around dusk. Feeding them is a misdemeanor, so keep the Twinkies inside the car.

Guided canoe tours can be booked for trips through Watson's Hammock, a nature preserve, at the shared headquarters for the Key deer refuge and the Great White Heron Refuge. The friendly folk there can give directions to the Blue Hole, an artificial lake that attracts some pretty large alligators. Here, as along U.S. 1, the gators have right of way.

Key West

This is party city, and the party begins at the Mallory Square pier each day when tourists and locals come together in a rather circuslike ritual to celebrate sunset. It's OK to applaud. To the left, a man with a ponytail balances on a high wire. To the right, a man with a French accent urges cats to jump through hoops. In between, two tall, thin men gently hammer out an island melody on steel drums.

More entertaining, though, is the ever-changing crowd, many of whom tend to be rather anesthetized with demon rum by the time the sun slips over the edge of the world in a gaudy kaleidoscope of color. Arrive about an hour before sunset for the full effect.

The yuppie crowd may be found wasting away in Jimmy Buffet's Margaritaville bar on Duval Street. More get down-and-party types head for Capt. Tony's Saloon, the original Sloppy Joe's where Ernest Hemingway used to hang out. Capt. Tony's walls are papered with what must be thousands of business cards. Near the entrance hangs a toilet seat that frames a photo of the (you fill in the blank) of the week.

A 1 1/2-hour ride on the Conch (pronounced "konk") Tour Train gives a thorough introduction to Key West's major attractions, taking passengers along Duval Street (called the longest street in the world because it links the Atlantic and the Gulf of Mexico) to the southernmost point in the continental United States, through Key West's museum district, past Hemingway's house, out by the (man-made) beach and through the Turtle Kraals area with its funky shops and raw bar.

In Key West, snorkelers like exploring the shallow reef near Sand Key Lighthouse. Experienced divers head for the Hump, Eyeglass Bottom, Ten Fathoms Bar and Outside Reef, where depths hover near 50 feet, then drop off up to 200 feet at the wall, giving them a look at rare deep-water corals and schools of cow-nose or spotted eagle rays, mantas, amberjacks and even the occasional sailfish.

No alligators here, though, unless nuggets of fried alligator tail, a popular appetizer at Key West restaurants, count. But they do give pause for thought.

Just how effective are those alligator crossing signs?