Across the Potomac along I-395. Closest Metro Pentagon. Mon—Fri 9.30am—5pm, last tour 3.30pm. 703/695-1776. Admission free.
The headquarters of the US military establishment is one of the largest chunks of architecture in the world: though it's only five storeys tall, the total floor area of over 6.5 million square feet is three times that of the Empire State Building. Each of the five sides is over 900ft long, and the combined length of all the internal corridors totals up at over seventeen miles. These and other useless factoids are about all you get from visiting the behemoth building, apart from the opportunity to see at first hand the people responsible for spending billions of tax dollars on those proverbial $50,000 toilet seats. In the one novel departure from the norm, the service-personnel guides who accompany you walk backwards the entire time to ensure that disguised foreign agents don't slip off into the restrooms.
Ninety-minute guided tours leave every half-hour from the small waiting area inside the entrance; take photo ID.