A pentagon is a five-sided polygon.
The Pentagon is the headquarters of the United States Department of Defense. The pentagon-shaped building houses approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel.
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 only seven minutes to walk between any two points in the building.
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, and enough steel was conserved to construct an entire battleship.
As part of the September 11, 2001 Terrorist Attack, American Airlines flight 77 crashed into the building, causing part of it to collapse. For pictures and graphics showing the damage in the impact see this briefing.