Video: 'The Pentagon - Decades TV Network'

(Friday, January 15, 1943; during World War II) — The Pentagon, headquarters building of the United States Department of War (now Defense), was completed today only 16 months after ground was broken in Arlington County, Virginia.

The Pentagon is the world’s largest office building, with about 6.5 million square feet of floor space, of which 3.7 million square feet are used as offices.

Some 23,000 military and civilian employees, and another 3,000 non-defense support personnel, work in the Pentagon.

Video: 'THE PENTAGON (FULL INSIDE DOCUMENTARY)'

It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 miles of corridors. The central five-acre pentagonal plaza is nicknamed “ground zero” on the presumption that it would be a prime target in a nuclear war.

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 others: 59 on the airplane and 125 in the Pentagon.