The Pentagon is the headquarters of the U.S. Department of Defense.

At present, it is home to approximately 26,000 military and civilian employees and roughly 3,000 non-defense support personnel who work in its 3,705,793 square feet of dedicated office space.

The building, which derives its name from its geometric shape, contains 17.5 miles of corridors but because of its shape, it only takes seven minutes to walk between any two points.

To give some sense of the size, consider that the Pentagon has three times the floor space of the Empire State Building and it is big enough to fit the United States Capitol building inside any one of the its five wings. It contains 6,500,000 gross square feet of space and 7,754 windows.

Built on the 34 acre site of the old Hoover Airport, the building is constructed out of reinforced concrete made from 380,000 tons of sand and supported by 41,492 concrete piles.

At a final cost of 83 million dollars, the Pentagon was dedicated on January 15, 1943, nearly 16 months to the day after the groundbreaking ceremony on this day, September 11th, 1941.

The building was conceived at the request of Brigadier General Brehon B. Sommervell, Chief of the Construction Division of the Office of the Quartermaster General. It was needed to ease the War Department’s critical shortage of space at the time.