Place : Arlington.
Description : This building shelters the Defence Department of the United States of America. More than 26 000 people work there, civil and soldiers. Its nameis due to its pentagon form. This 5 floors building, inaugurated on January 15, 1943, is the vastest office building of the world, with its 14 mile (28 km) corridors. With its 5 concentric rings, the pentagone was built with reinforced concrete by a steel reinforcement. The construction of the Pentagon began on September 11, 1941.