Pentagon

At the beginning of the Second World War, the War Department of the United States was spread out over 17 different buildings. To improve the efficiency and solve the overcrowding of the growing War Department, Congress approved a plan in 1941 to build one large central facility, consolidating the offices of the Army, Navy, Air Force and 14 defense agencies.

On September 11, 1941 construction started at a large site across the Potomac river, in Arlington, Virginia. Army engineers used 380,000 tons of sand and gravel dredged from the river to create the reinforced concrete used for the massive building.
The design was straightforward but very efficient: a 5 acre (2ha) central courtyard is surrounded by five pentagonal rings. The five different wedges forming the pentagonal shape were created one by one and as soon as a wedge was finished, it would be occupied. The first occupants moved in the Pentagon in April 1942 and the building was completed in January 1943.

In a span of just 16 months, an immense building with a floor size of 6,6 million sq ft or 600,000 sq m was created. It was almost twice the size of the largest office building at the time, the Merchandise Mart in Chicago and has three times the office space of the Empire State Building in New York, then the tallest building in the world. The Pentagon has 17.5 miles / 28 km of corridors spread over 5 floors. Due to its efficient structure, it takes never more than 7 minutes to walk from one point to another in the building.

Guided tours were given every hour by military staff in full uniform, but since the attacks on September 11, 2001 the tours have been restricted to US Residents. Check out their current policy on the Department of Defense's own Pentagon website.