The History of Fort Jefferson: America's Unfinished Fortress in the Gulf

The History of Fort Jefferson: America's Unfinished Fortress in the Gulf

Seventy miles west of Key West, rising out of turquoise water where there should be nothing but ocean, sits one of the most unlikely structures in America. Fort Jefferson is a six-sided brick fortress built on a tiny coral island, and it's the largest brick masonry structure in the Western Hemisphere. It was never finished, never attacked, and obsolete almost before it was built. Its story is still one of the best in Florida.

Turtles, Not Water

Spanish explorer Juan Ponce de León reached these islands in 1513 and named them Las Tortugas after the sea turtles his crew caught there. Mariners later added "Dry" as a warning: there's no fresh water on any of the islands. For centuries the Tortugas were mostly known as a navigational hazard, a cluster of low keys and reefs sitting right along one of the busiest shipping routes in the Americas.

Why Build a Fort Here?

That shipping route was exactly the point. After the United States acquired Florida in 1821, military planners realized that whoever controlled the Florida Straits controlled traffic between the Gulf of Mexico and the Atlantic, including the vital trade flowing out of New Orleans and the Mississippi River. The Tortugas also offered a deep, protected anchorage where a fleet could shelter and resupply.

A lighthouse went up on Garden Key in the 1820s, and in 1846 construction began on a massive coastal fortress named for Thomas Jefferson. The design called for three tiers of gun positions and roughly 420 heavy cannons, making it one of the most heavily armed forts ever planned in the country.

Sixteen Million Bricks in the Middle of Nowhere

Building a fortress 70 miles from the nearest town was a logistical nightmare. Every brick, tool, and barrel of food had to arrive by ship. Over roughly three decades, an estimated 16 million bricks were hauled to Garden Key and laid by hand.

The workforce was a mix of hired laborers, many of them immigrants, and enslaved men leased from owners in Key West. Later, military prisoners were put to work as well. They labored through brutal heat, hurricanes, and outbreaks of disease.

The island itself fought back. Garden Key is built on sand and coral, not bedrock, and the fort's enormous weight caused its foundations to settle unevenly. Iron shutters set into the gun openings rusted and expanded in the salt air, cracking the brickwork around them. You can still see the damage today.

The Civil War: Held, but Never Tested

When Florida seceded in January 1861, Union troops moved quickly to reinforce Fort Jefferson, and it stayed in federal hands for the entire war. No shot was ever fired at it in anger.

The war did something worse to the fort than any enemy could: it made it obsolete. New rifled artillery proved it could batter masonry walls into rubble, and forts like Jefferson suddenly looked like relics. Construction dragged on, but the fortress was never completed.

The Gibraltar of the Gulf Becomes a Prison

Instead of a front-line stronghold, Fort Jefferson found a new purpose: prison. Its isolation made escape nearly impossible, and during and after the war it held Union deserters and other military convicts.

Its most famous inmates arrived in 1865. Four men convicted in connection with the assassination of Abraham Lincoln were sent to the fort, including Dr. Samuel Mudd, the Maryland physician who set John Wilkes Booth's broken leg. In 1867, a yellow fever epidemic swept through the garrison, killing dozens, among them the fort's own doctor and one of the Lincoln conspirators, Michael O'Laughlen. Mudd stepped in to care for the sick, and soldiers at the fort later petitioned on his behalf. President Andrew Johnson pardoned him in 1869.

Abandonment and a Second Life

Storms, disease, and shifting military priorities took their toll. The Army pulled out in the 1870s, and the fort was left to the elements. The Navy later used the Tortugas as a coaling station, and the USS Maine anchored nearby before sailing to Havana in 1898, where its explosion helped trigger the Spanish-American War.

In the early 1900s, the Carnegie Institution ran a marine biology laboratory on nearby Loggerhead Key, drawn by the same clear water and rich reefs that visitors come for today. Ernest Hemingway, living in Key West in the 1930s, made fishing trips out to the Tortugas as well.

From Fortress to National Park

In 1935, President Franklin D. Roosevelt designated Fort Jefferson a national monument. In 1992, the area was expanded and redesignated as Dry Tortugas National Park, protecting not just the fort but the surrounding islands, reefs, seabird colonies, and sea turtle nesting beaches.

The fort that was built to control the seas is now one of the least-visited national parks in the country, largely because you can only get there by boat or seaplane.

See It for Yourself

Walking through Fort Jefferson's arched casemates, climbing to the top tier, and looking out over the moat to open water is something photos don't capture. Guided tours on the island bring the history to life, and when you're done you can snorkel the moat wall or relax on the beach.

Getting a seat is the hard part. The ferry regularly sells out, especially in peak season. If your dates are full, sign up for Dry Tortugas Alerts and we'll let you know when seats open up.