Naval Advance Base Espiritu Santo or Naval Base Espiritu Santo, most often just called Espiritu Santo, was a major advance Naval base that the U.S. Navy Seabees built during World War II to support the Allied effort in the Pacific. The base was located on the island of Espiritu Santo in the New Hebrides, now Vanuatu, in the South Pacific. The base also supported the U.S. Army and Army Air Corps, U.S. Coast Guard, and US Marine Corps. It was the first large advance base built in the Pacific. By the end of the war it had become the second-largest base in the theater. To keep ships tactically available there was a demand for bases that could repair and resupply the fleet at advance locations, rather than return them to the United States. Prior to December 7th, Pearl Harbor was the U.S. fleet's largest advance base in the Pacific. Espiritu became capable of all aspects necessary to support the Fleet's operations from fleet logistics in fuel, food, and ammunition, to transport embarkation for combat operations or returning to the continental United States. The ship repair facilities and drydocks were capable of attending to most damage and routine maintenance. Had it not existed, ships would have had to return to Pearl Harbor, Brisbane, or Sydney for major repairs and resupply. The base became a major R and R destination for the fleet.

USS ABSD-4, later redesignated as AFDB-4, was a nine-section, non-self-propelled, large auxiliary floating drydock of the US Navy. Advance Base Sectional Dock-4 was constructed in sections during 1942 and 1943 by the Mare Island Naval Shipyard in Vallejo, California for World War II. With all ten sections joined, she was 927 feet long, 28 feet tall, and with an inside clear width of 133 feet 7 inches. ABSD-4 had a traveling 15-ton capacity crane with an 85-foot radius and two or more support barges. The two side walls were folded down under tow to reduce wind resistance and lower the center of gravity. ABSD-4 had 6 capstans for pulling, each rated at 24,000 lbf (110,000 N) at 30 ft/min (0.15 m/s), 4 of the capstans were reversible. There were also 4 ballast compartments in each section.