Fort Jef­fer­son is the largest coastal fort in the Unit­ed States, locat­ed in the Flori­da Keys. This is a chain of about 1,700 coral islands and reefs in the south­east­ern Unit­ed States, on the ter­ri­to­ry of which sev­er­al nation­al parks are locat­ed: Ever­glades, Bis­cayne and Dry Tor­tu­gas. The con­struc­tion of the fort began in 1846 and last­ed about 30 years, but was nev­er com­plet­ed.