Base aérienne 921 Frères Mahé de Taverny is located in the communes of Taverny and Bessancourt in the Val-d'Oise department, north of Paris. The base was established in 1963 and served as a critical command center for French military operations. Its history dates back to 1941-1944 when Germans converted gypsum quarry tunnels for industrial use during World War II. In 1946, the French Air Force began using the quarries as a material depot. Key command centers were successively established, including the Centre d'Opération de la Défense Aérienne (CODA) in 1957, followed by the Commandement Air des Forces de Défense Aérienne (CAFDA) and the Commandement des Forces Aériennes Stratégiques (CFAS) in 1963. The Centre d'opérations des forces aériennes stratégiques (COFAS) was installed in the underground facility in 1967. In 1988, the base was named 'Frères Mahé' in honor of three brothers who died in service. The Commandement des Opérations Spéciales (COS) was located there in 1992 but relocated in 2006. Other units, such as the Centre de Conduite des Opérations Aériennes (CCOA), also moved out in 2007. The base was officially dissolved on July 5, 2011, becoming an 'élément air rattaché' (EAR) to Base Aérienne 110 Creil, though a residual nuclear alert activity was maintained. It was reactivated on September 4, 2024, with the Centre d'opérations des forces aériennes stratégiques (COFAS) and approximately 200 military personnel. The base is primarily underground, located in former gypsum quarries seventy to eighty meters below the Montmorency forest. It does not have a runway for fixed-wing aircraft but features a heliport. The underground facilities are designed for autonomy and protection against various threats, including electromagnetic pulses, housing a large Faraday cage. The base previously hosted the Commandement des forces aériennes stratégiques (CFAS), the Centre d'Analyse et de Simulation pour la Préparation des Opérations Aériennes (CASPOA), the Direction de la Circulation Aérienne Militaire (DirCAM), and the Centre de Météorologie des Opérations Extérieures (CMOE). The CFAS is responsible for the air component of France's nuclear deterrence.