14 ha The Pentagon Zone is a federal enclave and military reservation in Arlington County, Virginia, United States. It includes most of the District of Columbia within the boundaries of the state of Virginia. The Pentagon City station on the Washington Metro serves as the eastern terminus for the Blue and Orange Lines. The Pentagon Station is also the southern terminal for the Green Line of the Washington Metro. The Pentagon station opened on October 27, 1991, and is the first underground metro station in the Washington metropolitan area. It was built by the Metropolitan Transit Authority (METRO) as part of the Army's new headquarters building, which is next to it. The station lies beneath Massachusetts Avenue NW between E Street NW and Jefferson Drive NW. The Pentagon has one of the largest office buildings in the world. It is a 1.1 million square foot (104,000 m²) complex with eight floors above ground and seven below. It is the home of the United States Department of Defense. The Pentagon was originally constructed as a joint United States Army/United States Navy facility.