Four hours is hardly enough time to visit the tiny chunk of the Dry Tortugas that holds Fort Jefferson.</p><p>The Keys’ landmark attraction, famous as it might be, is arguably America’s least accessible national park. The round-trip time clocks at just under five hours on the Sunny Days boat the Fast Cat, though the trip time can be cut down by taking a seaplane.</p><p>The trip out can be positively boring. On a nondescript Saturday morning, more than half the passengers sleep, or at least try. Only a half-dozen Boy Scouts — who seemed to use the boat’s narrow hallways as a track — never tired. </p><p>With half the day dedicated to travel, visitors can only spend limited time on the sandy island that holds the fort. After a tour and lunch, that time is again cut down, now to only about two hours of free time to explore the island. And the boat’s crew instills a fear of missing the boarding deadline only the brazen would ignore. With no clocks and no cell phone service, a watch — a hot commodity among Saturday’s group — is a must for enjoying the Tortugas.</p><p>More serious explorers can camp, but the experience is not for all. A travel Web site’s first words about the smattering of campsites there: “this is a primitive camping experience,” with no amenities except saltwater toilets.</p><p>But don’t mistake time constraints for a ruined trip. The fort is one of a kind — literally: the building is the largest masonry structure in the Western Hemisphere. </p><p>The three-story brick walls are imposing, and still look plenty defensible after being out of use by the military for 100 years. Though, as a tour guide reveals, parts of the fort need repaired. (Indeed, some portions are closed for repairs, but that should not affect the quality of the trip.) </p><p>And the repeating rifle, which made the cannon-laden fort irrelevant could easily tear the building apart, visitors are told. Automatic weapons, as we call the repeating rifle, had not yet been invented when the fort was built.</p><p>Visitors coming on the Fast Cat have a ship crewmember pull double duty as story-spinning tour guides — a task they complete admirably well. </p><p>A Civil War history buff aboard Saturday’s Fast Cat tour found trolling around the cavernous fort riveting enough to eat up his post-tour free time. Most everyone else took to snorkeling, or at least attempting it. Winter waters, even on 80-degree days, feel frigid at first. Indeed, national oceanographic data puts them at less than 70 degrees in January.</p><p>A National Park Service map available on Wikipedia.org provides details about snorkeling hotspots, including the former coaling docks, which provide the best snorkeling but are not visible from the beach.</p><p>Aside from history and underwater fun, the whole park has proven an important bird sanctuary. Magnificent frigatebirds glide overhead every few hours, almost never flapping their wings. </p><p>The island is home hundreds of birds, according to the National Park Service. Among them are the sooty tern, which uses the Tortugas as an important nesting spot, and the masked booby, which looks like a photo negative of a penguin. </p><p>Cubans who make successfully landings leave behind patchwork boats that sit like large pita bread halves with rust-colored engines inside. Park staff must then work to dismantle the shoddy boats. </p><p>Tours include a word of warning about these immigrants. Interacting with the overjoyed and likely dehydrated Cubans is off-limits as rangers assess the situation. National Park rangers are federal police who “must react as if Osama Bin Laden himself has just landed.”</p><p><b>History of Fort Jefferson</b></p><p>“Arid turtles” might seem an odd descriptor for a group of sandbars, but in 1513 the Dry Tortugas earned that moniker. </p><p> Spanish Conquistador Juan Ponce de Leon found no fresh water there — no puddles, streams or wells, and certainly not the fountain of youth he sought. But hundreds of turtles populated the conglomerate of islands — an important food source that would keep aboard a ship for days, as upside-down turtles could live for days without escaping.</p><p>Fast forward to the colonial wars, when America contemplated protecting its borders for the first time. Spain owned Florida, a lawless wilderness. America bought the land in 1822, and sent an explorer down to evaluate putting a base in the Tortugas to fortify the Gulf and protect New Orleans.</p><p>The first commodore said the idea was terrible, and that fortifications would sink. Congress, not satisfied, sent a company man to dream up the base they wanted. He did just that, and they spent decades erecting a fort that could fire in all directions and house a few thousand soldiers and prisoners.</p><p>With no water available, the fort first had cisterns. Those soon cracked as the fort weighed down the sandbar island, which began sinking. A reverse osmosis plant was built within the walls to filter brackish water, which still functions today.</p><p>Every 25 feet or so both bottom floors contained small windows covered with cast-iron doors designed to be opened by cannon fire, and then automatically close. The fort also featured a furnace to heat cannon balls into glowing red “hot shots” that would do more damage to enemy ships. </p><p>The furnace still stands today, though the cast-iron doors have almost all rusted away.</p><p>Life on base would have been tough for Union soldiers outfitted with grey wool uniforms and forced to work in the 110-degree breezeless casemates — as each room within the walls outfitted with a cannon were known. Water was always scarce, supplies arrived late or not at all, and dehydration and illness killed regularly. It was a lonely life for the mostly New England-born crew.</p><p>The base became a Union prison during the Civil War. The most famous soldier was a doctor convicted of conspiracy in Abraham Lincoln’s assassination. Samuel Mudd helped heal John Wilkes Booth’s injured leg after Booth escaped from Ford’s Theater. Mudd’s descendents maintain his claim of innocence. </p><p>Mudd would later be pardoned, perhaps because of his work comforting yellow fever patients and stemming the disease’s spread.</p><p>As the 1800s wore on, the Army found the fort more costly than useful. Construction never finished for fear the weighty fort would sink the island. It became a quarantine facility, then was designated a wildlife refuge in 1908. The cannons never fired in the throes of battle, only for testing and training.</p><p><i>— History compiled from University of Miami archives and the National Park Service</i>