The Pentagon is the headquarters building of the United States Department of Defense, in Arlington County, Virginia, across the Potomac River from Washington, D.C. It was constructed on an accelerated schedule during World War II. As a symbol of the U.S. military, the phrase The Pentagon is often used as a metonym for the Department of Defense and its leadership. 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 impetus to gain Congressional approval for the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army Corps of Engineers, which supervised it. The Pentagon is the world's second largest office building, with about 6.5 million square feet (600,000 m2) of floor space, of which 3.7 million square feet (340,000 m2) are used as offices. 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, with a central five-acre (2.0 ha) pentagonal plaza. About 23,000 military and civilian employees work in the Pentagon, as well as about 3,000 non-defense support personnel.In 2001, the Pentagon was damaged during the September 11 attacks. Five al-Qaeda hijackers flew American Airlines Flight 77 into the western side of the building, killing themselves and 184 other people: 59 on the airplane and 125 in the Pentagon. It was the first significant foreign attack on federal facilities in the capital area since the burning of Washington during the War of 1812. Following the attacks, the western side of the building was repaired, with a small indoor memorial and chapel added at the point of impact. An outdoor memorial dedicated to the Pentagon victims of 9/11 opened in 2008. Source: Wikipedia (en)