Fort Jefferson , Florida Fort Jefferson is a massive but unfinished coastal fortress . It is the largest masonry structure in the Americas , and is composed of over 16 million bricks . The Dry Tortugas are part of Monroe County , Florida , United States . The fort is located on Garden Key in the lower Florida Keys within the Dry Tortugas National Park , about west of the island of Key West . Geography . It is located at , its elevation . History . Construction . In late December 1824 and early January 1825 , about five years after Spain sold Florida to the United States for $ 5 million , U . S . Navy Commodore David Porter inspected the Dry Tortugas islands . He was on the lookout for a site for a naval station that would help suppress piracy in the Caribbean . Unimpressed with what he saw , he notified the Secretary of the Navy that the Dry Tortugas were unfit for any kind of naval establishment . He reported that they consist of small sand islands a little above the surface of the ocean , have no fresh water , scarcely enough land to place a fortification , and in any case are probably not solid enough to bear one . While Commodore Porter thought the Dry Tortugas were unfit for a naval station , others in the U . S . government thought the islands were a good location for a lighthouse to guide ships around the area ' s reefs and small islands . A small island called Bush Key , later called Garden Key , was selected as the site for the lighthouse , which became known as Garden Key Light . Construction began in 1825 and was completed in 1826 . The 65 - foot lighthouse was constructed of brick with a whitewashed exterior . A small white cottage for the lighthouse keeper was constructed beside the lighthouse . In May 1829 , Commodore John Rodgers stopped at the Dry Tortugas to evaluate the anchorage . Contrary to Commodore Porter ' s experience , Rogers was delighted with what he found . The Dry Tortugas , he reported , consisted of 11 small keys and surrounding reefs and banks , over which the sea broke . There was an outer and an inner harbor . The former afforded a safe anchorage at all seasons , and was large enough to let a large number of ships ride at anchor . Of more importance , the inner harbor combined a sufficient depth of water for ships - of - the - line , with a narrow entrance of not more than 120 yards . Rogers said 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 . However , if occupied and fortified by the U . S . , the Dry Tortugas would constitute the " advance post " for a defense of the Gulf Coast . A series of engineering studies and bureaucratic delays consumed the next 17 years , but the construction of Fort Jefferson ( named after the third President , Thomas Jefferson ) was finally begun on Garden Key in 1846 . The new fort would be built so that the existing Garden Key lighthouse and the lighthouse keeper ' s cottage would be contained within the walls of the fort . The lighthouse would continue 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 . The original brick lighthouse tower was taken down in 1877 . Design . The design called for a three - tiered six - sided 410 heavy - gun fort , with two sides measuring 325 feet , and four sides measuring 477 feet . The walls met at corner bastion s , which are large projections designed to allow defensive fire along the faces of the walls they joined . The heavy guns were mounted inside the walls in a string of open casemates , or gunrooms , facing outward toward the sea through large openings called embrasure s . Fort Jefferson was designed to be a massive gun platform , impervious to assault , and able to destroy any enemy ships foolhardy enough to come within range of its powerful guns . Living quarters for soldiers and officers , gunpowder magazines , storehouses , and other buildings required to maintain the fort were located on the parade ground inside the fort ' s massive brick walls . The Army employed civilian machinists , carpenters , blacksmiths , masons , general laborers , the resident prisoner population , and slaves to help construct the fort . By 1863 , during the Civil War , the number of military convicts at Fort Jefferson had increased so significantly that slaves were no longer needed . At the time , there were 22 black slaves employed on the project . Fort Jefferson ' s peak military population was 1 , 729 . In addition , a number of officers brought their families , and a limited number of enlisted personnel brought wives who served as laundresses ( typically four per company ) . There were also lighthouse keepers and their families , cooks , a civilian doctor and his family , and others . In all , there were close to 2 , 000 people at Fort Jefferson during its peak years . 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 . Sand - filled columns were placed at regular intervals in the inner walls , spanning their height from the roof to the foundation . The columns were intended to filter rainwater from the rooftop for long - term storage in a series of underground chambers . However , the system was never used in practice , as the enormous weight of the outer walls caused them to subside ; this created cracks in the cisterns , allowing seawater to contaminate the fresh water supply . Active use : 1860s – 1930s . The fort remained in federal hands throughout the Civil War . With the end of hostilities in 1865 , the fort ' s population declined to 1 , 013 , consisting of 486 soldiers or civilians and 527 prisoners . The great majority of prisoners at Fort Jefferson were Army privates whose most common transgression had been desertion , while the most frequent transgression for the civilian prisoners was robbery . However , in July 1865 four special civilian prisoners arrived . These were Dr . Samuel Mudd , Edmund Spangler , Samuel Arnold , and Michael O ' Laughlen , who had been convicted of conspiracy in the assassination of President Abraham Lincoln . Construction of Fort Jefferson was still under way when Dr . Mudd and his fellow prisoners arrived , and continued throughout the time they were imprisoned there and for several years thereafter , but was never completely finished . Mudd provided much - praised medical care during a yellow fever epidemic at the fort in 1867 ( which killed many prisoners including O ' Laughlen ) , and was eventually pardoned by President Andrew Johnson and released . By 1888 , the military usefulness of Fort Jefferson had waned , and the cost of maintaining the fort due to the effects of frequent hurricanes and the corrosive and debilitating tropical climate could no longer be justified . In 1888 , the Army turned the fort over to the Marine Hospital Service to be operated as a quarantine station . Park designation . On January 4 , 1935 , President Franklin D . Roosevelt , who visited the area by ship , designated the area as Fort Jefferson National Monument . It was listed on the National Register of Historic Places on November 10 , 1970 . On October 26 , 1992 the Dry Tortugas , including Fort Jefferson , was established as a National Park . The islands still do not exhibit any standing fresh water or even seasonal streams , hence the name " dry " . Owing to the potential difficulties of survival in such conditions , one of these islands was used as the location for filming a military survival film used to train aircraft personnel .