In 1917, when the Marine Expeditionary Force in the Pacific was assigned to Mare Island, the MINSY Marine Corps compound, which was always under Marine Corps command, was relocated to the large hilltop isolated from and west of Shipyard South. The newly built Building M37, a handsome three-story reinforced-concrete building of Beaux Arts design, occupied the entire eastern end of a huge parade ground. It served as the focal point, headquarters, and barracks of the Marine Corps detachment at MINSY. During World War I, the MINSY Marine Corps compound was the major training station on the West Coast. In July 1917, MINSY was the site of a major explosion that killed six people. On July 9, a gunpowder magazine containing 127,600 pounds of black powder blew up, damaging a number of surrounding buildings, and leaving a mystery as to what had caused it. Suspicion settled on an identified German agent and possible saboteur,
Lothar Witzke, but the investigation proved inconclusive and the official verdict was that the cause was unknown. Stephen C. Ruder has suggested in a 2022 article that it may not have been an act of German sabotage but suicide by a civilian, Neil Damstedt, who was the principal victim and only individual inside the magazine at the moment of explosion. MINSY saw major shipbuilding efforts during
World War I. MINSY holds a shipbuilding speed record for a destroyer that still stands, launching in just days in May–June 1918. Mare Island was selected by the Navy for construction of the only US West Coast-built
dreadnought battleship, , launched in 1919. Several
pre-dreadnought battleships had previously been launched at San Francisco and
Seattle. Noting the power of
underwater warfare shown by
German U-boats in World War I, the Navy doubled their Pacific-based
submarine construction program at
Puget Sound Naval Shipyard by founding a submarine program at MINSY in the early 1920s.
Mare Island Marines Football Team During this period, the Marines stationed at MINSY fielded a
college football team which competed against teams in the
Pacific Coast Conference and other military service football teams. After the United States entered
WWI during April 1917, many college athletes joined or were drafted into the military. Enrolments diminished at universities around the country, with many schools cutting back their sports programs as a result. Consequently, military academies and training bases established football teams composed mostly of college players who had been recruited for service. This was further encouraged by officials who considered football to be excellent war training, with its themes of leadership, teamwork, and discipline. The
1917 Mare Island Marines football team strung together an impressive first season, going 8–0, shutting out six opponents, winning the
1918 Rose Bowl against the
Camp Lewis 91st Division football team, and outscoring all of their opponents by a combined total of 200 to 10. This was despite only 500 Marines being stationed at the base when the team was formed. The
1918 Mare Island Marines football team fielded an entirely new roster, which achieved a similar level of success as the previous year. They finished the season with a 10–1 record, losing only in the
1919 Rose Bowl to the
Great Lakes Navy Bluejackets.
Interwar years , with the Marine Barracks (Building M37) in background The Marine Corps training continued after World War I until 1921, when the Marine Corps recruitment center opened in San Diego. The MINSY Marine Corps detachment continued to occupy Building M37 until 1996, but never regained its pre-1921 importance. In 1919, MINSY was finally, after 65 years, linked to mainland Vallejo by a
causeway with a drawbridge, which provided direct land-based movement of supplies and personnel across Mare Island Strait. Owing to the shallowness of the shipping channels in Mare Island Strait and San Pablo Bay, the larger battleships and carriers of the Pacific fleet were sent not to MINSY but to leased dry docks then-privately owned at
Hunters Point. However, submarine work was especially suited for Mare Island. MINSY became the major West Coast submarine repair facility in WWI and in 1925 was awarded a contract to build its first submarine, . In the 1930s, land reclamation nearly doubled the usable acreage on Mare Island. For example, the low-lying North End marshland were raised above high tide, making it available for future MINSY expansion. Before World War II, the Navy established Station I at Mare Island as one of four High Frequency Direction Finding (HFDF) stations on the Pacific mainland to track Japanese naval and merchant shipping east of Hawaii. The other stations were: Point Arguello, California (Station Z), Point Saint George, California (Station T), and Fort Stevens, Oregon (Station S). ==World War II==