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 11 September 1941, and the building was dedicated on 15 January 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 6.5 million square feet (150 acres; 60 ha) of floor space, of which 3.7 million sq ft (85 acres; 34 ha) 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 miles (28.2 km) of corridors. The central five-acre (2.0 ha) pentagonal plaza is nicknamed “ground zero” on the presumption that it would be a prime target in a nuclear war.
Pentagon tours must be reserved at least 14 days in advance and no more than 90 days in advance. Tours are conducted Monday-Thursday (excluding federal holidays) from 10 a.m.- 4 p.m. and from 12 – 4p.m. on Fridays. Tours fill up quickly and it is highly recommended that you book well in advance of your visit.
Tours are 60 minutes and cover about 1.5 miles inside the Pentagon, which is one of the largest office buildings in the world. Tours include the history of the four branches of the military and the opportunity to see the indoor memorial near the Sept. 11 crash site and the Sept. 11 Memorial chapel, as well as the Hall of Heroes (featuring the names of all the Medal of Honor recipients) and a number of other military displays.
According to en.wikipedia; washington.org. Source of photo: internet