Landing is permitted on both islands (use kayak landing/launch areas, not the government docks), and rangers provide tours twice daily, Thursday through Monday. On Lignumvitae, there's a grassy area for picnicking, a pit toilet, the 1919 Matheson historic house museum and a nature trail through the virgin forest. Beware the mosquitoes.

MIDDLE KEYS

The view from high atop Channel 5 Bridge as you cross from Craig Key to Long Key seems the epitome of the Keys ideal: remote islands framed by Caribbean blue water. Ideals aside, this snapshot captures nicely the transition from Upper to Middle Keys.

From here, the main islands taper into a narrow, linear chain. For 33 miles -- across Long Key, Grassy Key, Key Vaca and a host of smaller islands set in between -- are points where only a few hundred yards of land separate ocean and bay. Instead of the nearshore mangrove islands that typify the Upper Keys, there is more open water and hardbottom communities. Swaths of white sand bottom frame striking orange sea stars and burnt-red sea cucumbers.

The dearth of sheltered paddling makes areas like Long Key Lakes, inside Long Key State Park, all the more treasured. Here, kayaks and canoes glide inches above a silty bottom replete with Cassiopeia jellyfish. Scores of tiny killifish create glittering silver rainbows as they jump from the water.

Amid the Whisky Creek mangroves in the heart of Marathon's Boot Key, narrow creeks link three interior lakes. Mullet jump in frenzied fashion as you push through a break of mangrove branches into yet another shallow, seagrass-lined ''room.'' A circuit through this wonderland clocks in at only two miles, but it takes a full day of paddling to soak in the beauty.

If open water distinguishes the Middle Keys, the crossing from Long Key to Conch Key is its best display. From a boat you can see up close what you can't in a car: The architecture of the Long Key Viaduct. In name and appearance, this bridge conjures up Romanesque grandeur. In a bygone era, the 186 ''spandrel arches'' across 2 ½ miles of open water carried passenger rail cars on a narrow track 30 feet above the water. The bridge was the ''first completed triumph'' of Henry Flagler's Overseas Railroad, writes Pat Parks in her book, The Railroad that Died at Sea.

With the coming of the railroad, the Florida Keys took on a new identity, one far removed from that of an isolated archipelago suitable for only the toughest and hardiest pioneers. Flagler built the Florida East Coast Railroad's Key West Extension to link Miami with Key West, and to cash in on Cuba's proximity.

Long Key Fish Camp, it's been written, was the Florida Key's first ''resort.'' Built as a work camp, Flagler had the cabins converted into lodging for guests of the railroad. Postcard images of small seaside shacks set amid silver palms did much to boost the image of the Keys as a tropical paradise.

LOWER KEYS

The Lower Keys are nothing like what precedes it. With a healthy does of imagination, they appear on a map as if someone smeared them across shallow waters of the Backcountry. Islands are oriented northwest-to-southeast, divided by long, wide channels. Soft corals and sponges predominate in nearshore hardbottom environments. The channels, by contrast, are carpeted with turtle and manatee grass.

More than 200,000 acres of water and small islands make up the Backcountry. Birding is phenomenal in this vital nesting habitat for the namesake of the Great White Heron National Wildlife Refuge. Royal terns group on a sandbar near the Contents, intermingled with laughing gulls and the odd oystercatcher. Near the Mud Keys, osprey soaring high overhead issue their signature whistle as they scan the water for prey. White and brown pelicans, little blue herons, tri-colored herons, great egrets, snowy egrets, and a host of wading birds work the mangrove flats from Cutoe Key to Cayo Agua.

A string of islands starts at the Content Keys and runs southwest to include the Sawyers, Barracudas, Marvin, Snipe Point, Mud Keys and Lower Harbor Keys. A small reef abruptly marks the boundary between the Keys' shallow waters and the deeper Gulf of Mexico. It is an ambitious open water journey for a kayak, between five and seven miles from convenient put-ins. But a trip out to the edge of the nearshore waters is not soon forgotten.

Mainland Florida is a distant memory by the time you reach Big Pine Key, where some still live off the grid. Off the highway, places like the No Name Pub and Geiger Key Marina don't mimic someone's idea of the Keys -- they are the Keys: Good food and cold beer, and an ear you can bend with a story about that 12-foot shark that bumped the boat.

An old dog greets you at the Sugarloaf Airport, a favorite of movie directors filming Third World airport scenes. Within sight of the airstrip, Five Mile Creek takes you into a mangrove forest via a network of deep, crystal-clear creeks lined with sponges and starfish.

Remember that imaginary line on U.S. 1? It's been a few hours since you crossed it, and you've slipped into the quiet of a mangrove creek. Perhaps a manatee has gently bumped the underside of your kayak, or poked its grey snout and rough whiskers out of the water. Maybe you've been startled by the sudden whoosh of a stingray or small brown nurse shark swimming away. What's certain is you've never paddled in a place quite like Keys.

This article is based on the Burnhams' new FalconGuide, Florida Keys Paddling Atlas (Globe Pequot Press, 2007).

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