Historic Marker number 57 is located at the West end of Southard Street along with the markers for the USCGC Ingham Museum and the Outer Mole Pier.
What we know today as Truman Annex got its start in 1845 as part of the Army installation at Fort Taylor. The base was eventually taken over by the Navy in 1947 and named the "Fort Zachary Taylor Annex to Naval Station Key West." By 1932, five new metal "finger" docks had been added to the harbor to create a home for Navy submarines.
The submarine docks, dry docks and surrounding support buildings were very important during World War II. Due to an oversight in the Treaty of Versailles following World War I, Germany amassed the largest submarine fleet in the world. The submarines, or U-boats as they were commonly called, were very successful in inflicting great losses on Allied supply convoys in the Atlantic. During the war 3,000 Allied merchant and military ships were lost to U-boat encounters. While most effective in the mid-Atlantic where there was a large gap in air power, their threats were also felt on the shores of Florida. In May of 1942, German U-boats sank 47 ships off the shores of Florida. Often the burning wreckage could be seen from shorelines along the coast.
A number of different submarine missions were conducted from Key West's Naval base. Some involved hunting and destroying enemy ships in the Atlantic, Caribbean and Gulf Coast waters. Others entailed escorting ship convoys that sailed to and from England with crucial supplies for the European Front. Convoy escorts by Navy submarines and sailors are credited with saving countless lives at sea and abroad during the war.
The base also served to support the Pacific War against Japan. New submarines built in east coast shipyards were sent to Key West where they received their crews and orders to sail through the Panama Canal to take their place in the Pacific Ocean Theatre.
The Fleet Sonar School moved to the base in 1940 and was instrumental in training thousands of officers and sailors about newly evolving sonar technology and submarine warfare tactics. The use of sonar on ships was one of the few ways available to track the presence or movements of an enemy U-boat.
On March 20, 1946, the Naval Station was designated as the U.S. Naval Submarine Base Key West. Subsequently, nuclear-powered submarines replaced Key West's outdated diesel-powered vessels and the base was closed in 1974.