The Pentagon is the headquarters of the United States Department of Defense. The building is pentagon-shaped in plan and houses approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel in Arlington, Virginia. It has five floors and each floor has five ring corridors. The Pentagon was dedicated on January 15, 1943 and it remains the world's largest office building.
Built during the early years of World War II, it is still thought of as one of the most efficient office buildings in the world. Despite 17.5 miles of corridors it takes a maximum of seven minutes to walk between any two points in the building.
The central plaza in the Pentagon is the largest "no-salute, no-cover" area (where hats need not be worn and salutes are not required) in the world. It is informally called ground zero.
It was built from 680,000 tons of sand and gravel dredged from the nearby Potomac River that were processed into 435,000 cubic yards of concrete and molded into the pentagon shape. Very little steel was used in its design due to the needs of the war effort.
As part of the September 11, 2001 Terrorist Attack, American Airlines flight 77 crashed into the building, causing part of it to collapse and killing 125 people in addition to the 64 aboard the plane. For pictures and graphics showing the damage in the impact see this briefing. The first of the workers whose offices were destroyed or damaged in the attack began moving back in on August 15, 2002. 
Facts & figures