The Pentagon is the headquarters of the United States Department of Defense and symbol of the U.S. military. The building is so massive that it contains over 17 miles of hallway. Though home to many classified secrets, the Pentagon is open to the public through guided tours. The tours are free, but only available via reservations placed 14 to 90 days in advance.
The guides are impressive—they are chosen from the military's Ceremonial Guard, memorize a 35-page script, and walk the entire route backwards in order to maintain eye contact with visitors. Wear comfortable shoes, since the tour usually lasts about an hour and follows a circuit almost one-and-a-half miles long.
You'll have to pass through security, so arrive early and bring a valid ID and confirmation letter.