The Pentagon is the headquaters of the American Deparment of Defense. A visit to the Pentagon is the only way to appreciate the immense size of this building (the United States Capitol could fit inside any one of the Pentagons five wings) and a guided tour will take you through only a fraction of the 17.5 miles of corridors inside. "Walk-in" Tours begin at the Metro entrance and take approximately one and a half hours to complete the mile long route. Group tours are also available, but require advance booking. Note that photographic Identification is required.