Pentagon Building

Constructed as the United States mobilized before the Second World War, the Pentagon Building was, at the time, the largest office complex in the world. It remains the headquarters of the Department of Defense and the senior officials of the United States armed services, but the Department long ago exceeded the space inside the facility.

It is an unusual and quite efficient building design. There are actually five concentric pentagonal buildings, each with five stories above the street level. The outermost and most prestigious is the "E Ring"; the innermost, which surrounds a reasonably pleasant courtyard, is the "A Ring". For many years, a snack bar in the center of the courtyard was called the Ground Zero Cafe. It was torn down for a new structure, but will rise again.

Radiating from the center are numbered "corridors". A given room is identified by floor, ring, corridor, and room number using a standard reference from the lower-numbered corridor. For example, the Office of the Secretary of Defense is in suite 3E880:
 * Third floor
 * E Ring
 * 80th position from Corridor 8

Contrary to urban legend, there are no deep, hardened shelters under the Pentagon. There are several full and partial floors below ground, but, for example, the National Military Command Center is on the second floor. Prior to the threat of vehicle-borne explosives, a commuter bus station was on ramps below the building; these have been closed and converted to some of the more pleasant, if underground, offices.

Its site remains a major transportation hub for the area; the Blue and Yellow lines of the Metrorail subway system meet in the transportation complex outside, and there are extensive lanes for commuter buses just outside the subway entrance. There is a heliport for executive transport helicopters, and even a shuttle boat from Bolling Air Force Base and other facilities on the Potomac River.

Authorization, Location and Construction
While it is usually described as in Washington, DC, most of the building is in Arlington, Virginia; the Arlington County Fire Department was the incident commander for the fire and rescue response after the 9-11 attacks.

Construction was directed by then-brigadier general Leslie Groves, who finished it under budget and ahead of schedule. That accomplishment led to his next assignment of managing the Manhattan Project, the U.S. program to build the first nuclear weapons.

At the time of construction, the dividing line between the District of Columbia and the Commonwealth of Virginia ran down the center of the Potomac River. Since part of the Pentagon rests on land dredged from the Potomac, some of which crossed the then-center of the river, it is true that part of the building is in Washington proper.