During World War II, Casco Bay became United States Navy base "Sail" for destroyers escorting HX, SC, and ON convoys of the Battle of the Atlantic. Facilities constructed on the island included the Torpedo Control Officers School of the Portland Naval Training Center, a navy supply pier with a naval fuel annex often cited as a source of contamination for both the island's marsh and Casco Bay, and the Naval Auxiliary Air Facility Casco Bay seaplane base operated as part of Naval Air Station Brunswick from 14 May 1943 to 15 December 1946. The United States Army Coast Artillery Corps built two batteries of 90 mm dual-purpose guns on the island as the Long Island Military Reservation, part of the Harbor Defenses of Portland.