The Pentagon. Those two words are enough to conjure up images of “Top Secret” signs hanging on closed doors, like the kind shown in movies. Well, those doors are actually not as closed as you might think. By making a reservation in advance, the symbolic building known as the headquarters of the Department of Defense opens up via a guided tour.

The tour takes about 90 minutes and covers nearly two miles, visiting numerous displays and memorials that have had a significant place in our nation’s history. All the tours are free, and there’s an option to go it alone on a private tour or in groups of five or more. You can also visit the Pentagon Memorial, which honors the victims of 9/11, any day of the week without a reservation.

Just seeing the unique architectural design is an amazing experience as well. As the largest office building in the world, it’s virtually a city in itself with 23,000 employees, 4,200 clocks, 691 water fountains, 284 restrooms, two cafeterias and six snack bars located throughout the 6,5000,000 square foot floor area. The most amazing thing is that, despite having 17.5 miles of corridors, it takes seven minutes- at most- to walk between any two points in the building.