Divers still pluck plump Spanish emeralds from the ocean around here. And when the wind is high, ruffling the vivid aqua blue-green sea, the raucous cries of sooty terns set your senses atingle. They remind you of a pirate's taunting laugh‹a menacing cry issued by many a brigand who preyed on treasure-laden ships in these waters while sailing under the black flag‹bound for the Dry Tortugas.

Now I, too, was bound for these small coral-and-sand islands, cruising on the Yankee Freedom II to America's southernmost and perhaps most intriguing national park. Closer to Cuba than the United States mainland, the Dry Tortugas form one of our few island national parks and is rich in history and marine and bird life while possessing an intense high-color beauty.

The fact that these mid-ocean islets can only be reached by boat or seaplane adds to the sense of romance and mystery that engulf you as you set out on an excursion to these fabled land spits. To reach them, you depart from Key West and journey even farther west some 70 miles to the real end of Florida's chain of keys.

A 40-minute flight by seaplane, the national park can also be reached by a 2-1/2-hour ferry ride and I haunted the boat's observation deck eagerly scanning the horizon‹barely discernible between a cloud-speckled azure sky and similarly colored sea‹for a glimpse of brilliant white sand or a long, low slab of red brick. Garden Key, the largest of seven that make up Dry Tortugas National Park, was where we were headed. Almost its entire expanse, I had been told, was covered by history-riddled Fort Jefferson, once a garrisoned bulwark against pirates and unfriendly nations looking to cause trouble in the vital shipping lanes that run through the Gulf of Mexico and the Florida Straits.

Abruptly, a stubby sliver of darkness began to pry apart sea and sky and the unlikely sight of the largest masonry structure in the Western Hemisphere, squatting purposefully on a 16-acre subtropical island, hove into view.

The Yankee Freedom's complement of day-trippers crowded the 100-foot-long catamaran's port side to Goggle at the sun-slathered hexagonal fort, a sugar-white beach rimming it like a fancy dessert doily.

A few people tried to contact relatives on their cell phones hoping to detail the scene, but to no avail. Cell phones don't work out here‹the peaceful isolation reinforced by the fact that there are no facilities on the Dry Tortugas: no public telephones, restrooms‹you use the ones on the boat, nor shops, restaurants, snack bars or even fresh water. The Yankee Freedom brings along a picnic spread which the crew serves buffet-style on the beach after a fort tour and time to wander. On the trip out, bushy-bearded Jack Hackett, the onboard naturalist, had already peaked the interest of passengers with Dry Tortugas' lore, and it was he who would lead a 45-minute tour of the fort. Park rangers, affable and knowledgeable, have their hands full protecting, patrolling and maintaining Fort Jefferson so the two cruise companies that run these popular day-trips out of Key West provide guides for the fort tours. (Collecting and caring for Cuban exiles, then getting them safely to Key West has added to park rangers' duties in recent years. For the refugees, after fleeing Cuba in their homemade boats or aboard the sleeker, faster craft operated by smugglers, land on the Dry Tortugas as these islands are the closest United States territory they can reach in their quest for a better life.)

So, in a way, history is still being made on these strategically placed isles. But Jack Hackett is the man to tell you about what went on in the past.

Hackett begins by answering the intriguing question: Why was the place named "Dry Turtles?"

It was that peripatetic Spanish explorer Ponce de Leon, it seems, who first discovered these islets in 1513 naming them Los Tortugas ("The Turtles") after catching over 100‹succulent fodder for his hungry crew.

Mariners who came later agreed about the turtles but were disheartened to find no fresh water and re-christened the coral reefs the Dry Tortugas.

Pirates held sway, according to Hackett, in the 1600s and 1700s, but it was in the 1800s that the American government realized just what a strategic spot this was. It decided to erect a fort that would protect the country's southernmost perimeter as well as ensure the safety of a huge and prosperous shipping trade that plied these waters. As I trailed our guide across the yawning parade ground‹today made less harsh with the addition of buttonwood and palm trees for shade‹and climbed up into the fort's three tiers, I was awestruck by the engineering feat accomplished in this isolated backwater.

Begun in 1846, construction‹never completed‹stretched over 30 years. All building materials had to be shipped in from the mainland including a whopping 16 million handmade red bricks. Eventually the walls would reach 45 feet in height (one reason you can spy the massive structure from a distance on the sea), while less visible but no less impressive were the fort's foundations‹14 feet wide and two feet thick‹which had to be erected under water.

In the 1860s, 2,000 souls were stationed or imprisoned here‹most unaccustomed to the scorching heat and made miserable by disease, scorpions, and, as one enlisted man wrote to his wife back home, grub that "has not been fit for a decent dog."

Jack Hackett gives us a visceral sense of these wretched conditions as he leads the group into one cramped cell that served as the dank, dreary quarters of the fort's most infamous prisoner.

Dr. Samuel Mudd, the hapless medic who set the broken leg of John Wilkes Booth, Abraham Lincoln's assassin, arrived here in 1865 to begin a life sentence. Cruelly treated, he still rose to the occasion when the fort doctor died during a yellow fever epidemic, and though also laid low by the deadly disease, Mudd labored valiantly to treat the fort's infected inhabitants.

The soldiers changed their minds about the selfless physician and petitioned President Andrew Johnson for his release; a pardon was duly issued Feb. 8, 1869 and Mudd was set free a month later. On a sunnier note, the fort was, at one point, designated a wildlife refuge saluting the area's rare marine and avian life before being christened a national monument in 1932.

It was not until 1992 that the Dry Tortugas was granted national park status in order to celebrate and protect the 100-square-mile park's historical and nature-based attributes. Some 95,000 tourists arrive annually to experience its unusual mid-ocean delights.

As I gazed out through a wide opening‹called an embrasure‹on the second tier, where one of the fort's 420 heavy guns once stood, a sparkling aquamarine sea met my eyes mellowing the harsh fact of this so-called "Guardian of the Gulf."

Looking down, it's easy to spy the moat‹still filled with emerald-green water‹that surrounds the edifice. The broad expanse of its outer retaining wall is a prime place to take a stroll enjoying the fort's red-brick bulk on the one hand, and the multihued coral and marine life of the sea on the other.

This is a snorkeler's paradise and just off the fort's moat, near the picnic area, visitors mass to float atop the sea and ogle this cornucopia of marine life. You can bring your own gear, but both ferry companies supply equipment free to passengers who, after their sandwich-and-salad picnic lunches, head for the water to swim and snorkel to their hearts' content.

If you've completed the fort tour (you can also do a self-guided tour following designated signs), and finally reached the third tier where the old iron lighthouse still stands, you want to gaze out on adjacent Bush Key‹cram packed with birdlife. Binoculars are in order as you drink your fill of a mind-boggling 100,000 sooty terns who nest in declivities in the sand from March to September in their only significant breeding colony in the Continental United States.

The Dry Tortugas lie in the midst of a major flyway running between South America, Cuba and the United States, and 299 bird species have been spotted here including ruby-throated hummingbirds, white-eyed vireos, masked and brown boobies and brown pelicans. In fact, hundreds of thousands of birders have traveled to the Dry Tortugas just to revel in the sight of this bounty of birdlife.

The legendary naturalist and artist John James Audubon visited in 1832 to observe and draw some of the rare avian species congregated on Bush Key. He arrived in the spring, one of the best times to see the massive colonies of nesting birds.

It's also fortunate that this is a perfect environment for sea turtles. The namesake of these islands are in danger of extinction due to centuries of overhunting and this protected habitat is one of the few places left on earth where they live unmolested. Green, loggerhead, hawksbill and leatherback turtles roam the Dry Tortugas hauling out on secluded beaches each summer to lay and bury clutches of eggs. During a swim, you may be lucky enough to catch a glimpse of a hulking, stoutly armored turtle motoring gentle through waving clumps of sea grass‹the rare ocean dweller happy, as are many, to be bound for the Dry Tortugas.

IF YOU GO

For more information, contact:

Dry Tortugas National Park; 305-242-7700; www.nps.gov/drto Open: All-year round during daylight hours. How to Get There:

Three tour operations service the Dry Tortugas‹two ferry boats and a seaplane company. All three have excellent Web sites offering a lot of information.

Yankee Freedom II; 800-634-0939; www.yankeefreedom.com Operates out of Key West. Departs 8 a.m. and returns 5:30 p.m. Also free guided tour of the fort and the use of snorkel equipment. Sunny Days Catamarans; 800-236-7937; www.sunnydayskeywest.com. Operates out of Key West. Departs 8 a.m. and returns 5 p.m. The cruise is 2 hours each way. Sea Planes of Key West; 800-950-2359; www.seaplanesofkeywest.com. Operates out of Key West International Airport. Morning and afternoon departures for half-day tours.

Additional Park Information of Interest:

Visitor Center: The Visitor Center, located inside the fort, is a good place to collect a free copy of the Dry Tortugas' bird checklist. In fact, it's an excellent stop to make when you first arrive for the staff runs an informative video, there are worthwhile exhibits on display, and good periodicals, t-shirts, pins, postcards and such are for sale.

Campsites: Some visitors like to experience the island in total solitude, only possible after the mid-afternoon departure of the ferries and seaplanes.

Restrooms: After 3 p.m., specially designed composting toilets are unlocked for use by campers.

Bush Key Walks: From February to mid-September this island is closed to visitors because of its vital importance as a bird rookery. The Other Five Keys: Long and Hospital Keys are closed year-round while Loggerhead, Middle and East Keys are open during daylight hours. These islands are a distance from Garden Key and tour operators don't go there.

Shipwrecks: Choosing to fly to Garden Key on the seaplane is an alluring idea if you want to catch glimpses of famous and not-so-famous shipwrecks lying below in the sun-dappled, shoal-filled waters. Over 200 wrecks have been charted here granting the park yet another accolade‹possessor of one of the richest concentrations of shipwrecks in North America.