The Mare Island Naval Ammunition Depot was the United States' first naval arsenal in the Pacific, manufacturing, storing, and transporting munitions for the U.S. Navy from 1857 to 1975. Founded in January, 1857, it eventually expanded to over 500 acres, including most of Mare Island itself and large areas added to the island's southern end by filling in the bay. Its heyday came during World War II, when thousands of military and civilian workers supported the war effort by producing munitions used by the U.S. Navy throughout the Pacific Theater. The depot remained active until 1975 and was selected for closure in April, 1996 under the Department of Defense Base Realignment and Closure program. Today, the site includes cultural, historic, scenic, and recreational resources protected by the Mare Island Shoreline Heritage Preserve, with significant features of the Mare Island National Historic District including the Navy's oldest cemetery in the West, workers' military and civilian housing, and some of the oldest munitions storage magazines. Portions are listed on the National Register of Historic Places and contribute to the Mare Island National Historic Landmark, while environmental cleanup and munitions clearing continue and some areas remain off-limits except by guided access.