The Pentagon is the headquarters building of the United States Department of Defense. As a symbol of the U.S. military, the phrase The Pentagon is also often used as a metonym for the Department of Defense and its leadership.
Located in Arlington County, Virginia, across the Potomac River from Washington, D.C., the building was designed by American architect George Bergstrom and built by contractor John McShain. Ground was broken on September 11, 1941, and the building was dedicated on January 15, 1943.
General Brehon Somervell provided the major motivating power behind the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army.
The Pentagon is the world's largest office building, with about 6,500,000 sq ft (600,000 m2) of space, of which 3,700,000 sq ft (340,000 m2) are used as offices. Some 23,000 military and civilian employees, and another 3,000 non-defense support personnel, work in the Pentagon.
It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km) of corridors.
According to en.wikipedia