Pentagon

The US Department of Defense is housed in what may be the world's biggest office building, which took just 16 months to build back in WWII. About 25,000 people work in the massive polygon, which has more than a dozen miles of corridors and five sides surrounding a 5-acre courtyard. But while the formidable edifice appears impenetrable, 184 people were killed here on September 11, 2001, when American Airlines flight 77 crashed into the side of the building. Just outside of the Pentagon is a tranquil memorial to these victims (including passengers of flight 77); the grounds consist of 184 benches engraved with a victim's name, shaded by 85 paperback maple trees. Since the attacks the Pentagon building is open only for prearranged group tours (be sure to call at least several weeks in advance or visit the website). Virtual tours are online at www.defenselink.mil/pubs/pentagon.