Fort Jefferson

A Fortress in the Sea

Fort Jefferson is the largest masonry structure in the Americas — a massive hexagonal fortress rising from a tiny island 70 miles west of Key West in the Gulf of Mexico. Construction began in 1846 to control navigation in the Gulf and protect one of the nation's most strategic deepwater anchorages. Though never completed and never attacked, the fort served as a military prison during and after the Civil War.

Building an Impossible Fort

The U.S. Army Corps of Engineers chose Garden Key, a scrap of sand barely above sea level, as the site for what would become a 16-million-brick colossus. Workers — including enslaved laborers — hauled materials by ship from Pensacola and the northeast. The fort was designed to hold 450 heavy guns across three tiers of casemates, capable of commanding every approach to the Gulf.

Construction dragged on for 30 years but was never finished. Advances in rifled cannon during the Civil War made brick forts obsolete before the walls were complete. By the 1870s, the Army largely abandoned the project.

Dr. Mudd and the Lincoln Conspirators

Fort Jefferson's most famous chapter came after Lincoln's assassination in 1865. Dr. Samuel Mudd, who set John Wilkes Booth's broken leg during his escape, was convicted of conspiracy and sentenced to life imprisonment at the fort. Three other conspirators joined him. Mudd's cell can still be visited today. He was eventually pardoned in 1869 after helping combat a yellow fever outbreak that devastated the garrison.

From Prison to National Park

The Army transferred Fort Jefferson to the Navy in 1874, then to the Marine-Hospital Service as a quarantine station. President Roosevelt declared it a national monument in 1935, recognizing both its architectural ambition and its wildlife — the Dry Tortugas are a critical stopover for migratory birds and home to thriving coral reefs. It became part of Dry Tortugas National Park in 1992.

What to Know Before You Go

Garden Key is remote. There is no fresh water, no food service, and limited shade. The ferry includes lunch, snorkel gear, and a guided tour. If you're camping, bring everything you need — including all water (one gallon per person per day minimum). Cell service is nonexistent; tell someone your plans before you leave.

The reward is worth the effort: crystal-clear water, world-class snorkeling right off the beach, an eerily beautiful ruin, and some of the darkest night skies on the East Coast.