In late December 1824 and early January 1825, about five years after Florida joined the United States, U.S. Navy Commodore David Porter inspected the Dry Tortugas islands.

He reported that they consisted of small sand islands a little above the surface of the ocean, had no fresh water, scarcely enough land to place a fortification, and in any case were probably not solid enough to bear one.

In 1829, under recommendations from Commodore John Rodgers, the survey ship Florida stopped at the Dry Tortugas to evaluate the anchorage.

Tattnall noted that if a hostile power should occupy the Dry Tortugas, United States shipping in the Gulf would be in deadly peril, and nothing but absolute naval superiority could prevail.

Robert E. Lee, then a Captain in the U.S. army, shared this opinion and in February 1845 penned a letter to Thomas Blake (Commissioner of the General Land Office) recommending the reservation of the Dry Tortugas for military use.

John G. Barnard then made a detailed reconnaissance in November 1844 and on 17 September 1845, the Dry Tortugas became a national military reservation.

Workers consisted mostly of enslaved people hired from their owners in Key West and other parts of the State of Florida.

The lighthouse continued to serve a vital function in guiding ships through the waters of the Dry Tortugas Islands until the current metal light tower was installed atop an adjacent wall of the fort in 1876.

Corner bastions, which are large projections designed to allow defensive fire along the faces of the walls they joined, contained gunrooms, gunpowder magazines and a granite spiral staircase.

The 13-acre (5.3 ha) parade ground contained additional powder magazines, headquarters, a hospital, officer quarters and three large barracks.

[5] The Army employed civilian carpenters, masons, general laborers, and Key West slaves to help construct the fort.

[5]: 16–19, 23–27 In order to support such a large population in an area lacking fresh water, an innovative system of cisterns was built into the walls of the fort.

However, the rainwater dissolved salts in the sand, or the cisterns had not been made tight, making the water unfit for drinking, but usable for washing and cooking.

Artillery Regiment, under the command of Major Lewis Golding Arnold, were moved to the fort, preventing it from falling into the hands of rebel forces.

Meigs took over as the Superintending Engineer in 1860, and worked feverishly to improve the security and defenses so that the fort's heavy guns were first fired on 26 January 1861.

[5]: 49, 55–57, 63–65, 78–79 In September 1861, the first prisoner soldiers appeared, those sentenced by courts-martial to confinement and hard labor for acts such as mutinous conduct.

By November 1863, the number of military convicts reached 214, meeting the demands for unskilled labor, and the ratio of soldier to prisoners was about four to one.

This led to his detention in the fort's "dungeon," over which were the words "Whoso entereth here leaveth all hope behind" (from Canto III of Dante's Inferno).

[4] Frequent hurricanes and yellow fever epidemics convinced the War Department to remove the garrison, leaving a small caretaker force for the armaments and ammunition in 1874.

Neglected, stripped by vandals, swept by repeated tropical storms that crushed brick and concrete and bent girders, Fort Jefferson deteriorated rapidly.

Visitors by ferry typically spend four hours on the island, which is enough time for a guided tour of the fort, lunch on the boat and a swim (snorkel equipment provided) on the reef.