Urban explorer Thomas Kenning takes you on a guided tour of the most stunning abandoned fort in the Western Hemisphere. This is an extended excerpt from Sunshine Sentinels — a guided photographic tour of Florida’s abandoned coastal defenses. Available now from all fine booksellers.

Seventy miles beyond Key West lies a sandy chain of islands devoid of any freshwater. These desert islands make an ideal sanctuary for hundreds of nesting sea turtles and ocean birds. Collectively, they are the aptly-named Dry Tortugas. The shallows that dominate the floor of the Gulf between here and Key West are a natural minefield, slashed where small boats have run aground and scarred with the shipwrecks of those possessed of a deeper draft and less luck. In centuries past, most ships in transit between the northern Gulf and the Atlantic would naturally hew to the navigable passage of deep water that brings them within a cannonball’s shot of these lonely keys.

Thus, the quixotic Fort Jefferson, a floating fortress, Florida’s first theme park — a military fantasia in brick and mortar, comprised of sixteen million blocks forming a two-tiered hexagonal structure complete with firing positions for 420 large guns, arranged so that 125 of them could zero on the same target simultaneously. Its eight-acre parade had room for two massive powder magazines, a three-story thousand-man barracks, officer quarters, and a hospital. There’s also a hotshot furnace for heating cannonballs until they glowed red — the better to ignite the last generation of wooden warships, still in use when this fort was conceived.

When you see it from the air, before the window of your seaplane is misted over with a salty spray, when you can take the whole thing in at once, Fort Jefferson is an unrivaled colossus. Pure military might astride tiny Garden Key, an awesome American island in command of a wide-open sea — a projection of power that you can’t help but believe.

There’s the theme for your park.

There is exactly nothing understated about Fort Jefferson, a structure that seems to have little regard for the laws of nature.

Built over the course of three decades as a part of the Third System of coastal fortifications, which itself was an ambitious but awkward adolescent phase of the United States’ military ascendency, Fort Jefferson is more stagger than swagger. Its intricate cistern system never worked properly and actually managed to turn its collected rainwater salty. As at other forts in Florida, yellow fever was a serious issue from the moment that work crews of enslaved laborers arrived on Garden Key in 1846.

Even as you stand in the overgrown grass amidst cracked foundations on the parade, it’s hard to believe the scale of their work — Fort Jefferson’s two shortest curtain walls measure 325 feet in length, while the remaining four stretch to 477 feet. Most guns are long gone, but the 303 open-vaulted casemates remain, facing toward the sea through tattered embrasures.

Fort Jefferson is audacious in scale. It demands to be shot in widescreen. Vertically it can be described in geological terms.

Into those walls, the sediments of history — the coming of the Civil War, the abolition of slavery, the rise of wage labor — are demarcated as the epochal events that they were. Like a layer of ash from the catastrophic eruption of some ancient volcano, you can see where, four-fifths of the way up the wall, the jaundiced brick produced by enslaved hands in Pensacola gives way to a ruddy variety imported from Maine, baked by anonymous European immigrants and laid by Confederate prisoners of war.

The fort was largely stripped of its heavy guns and abandoned to the elements by the 1870s. A small caretaker force maintained the hospital as a quarantine station. During this period, it also served as a convenient coaling station for the navy’s Caribbean operations. In fact, in 1898, Fort Jefferson supplied the USS Maine with the coal that would blow it sky high over Havana harbor just days later, igniting the Spanish-American War.

In 1908, the Dry Tortugas became a national wildlife refuge, and in 1992, a national park.

Until recently, you could walk around the entire fort on top of the counterscarp, or moat wall. Successive hurricanes have since shattered that wall, making a complete circuit possible only with snorkeling gear. Even as the National Park Service undertakes conservation projects on features of Fort Jefferson like the Garden Key Lighthouse, it’s not hard to imagine a day in the near future when it’s unsafe to climb up to the barbette tier.

Fort Jefferson is a relic, a brick-and-mortar cul-de-sac of war-making technology from a time before large caliber rifled artillery could penetrate such brittle walls, before a changing climate threatened to swamp the whole unrealistic scene.

Get on a seaplane, take a ferry — get to Fort Jefferson before antebellum becomes antediluvian.

Thomas Kenning’s new book Sunshine Sentinels — a guided photographic tour of Florida’s abandoned coastal defenses — is available now from all fine booksellers.

Sunshine Sentinelsis a stunning visual survey of Florida’s historic coastal defenses — those sun-faded outposts of empire, crumbling now on shifting sands near the end of the beach. It is a guided tour in book form, perfect for these stay-close-to-home kind of times! Seven thrilling tales, supplemented by 140 original images reproduced at the highest quality, providing a view of the Sunshine State that rarely makes the postcard.

You can find additional an additional preview of Sunshine Sentinels on his website, thomaskenning.com.

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