The Pentagon and Pentagon Memorial

Headquarters of the US Department of Defense and the largest office building in the world, across the Potomac in Arlington, Virginia. Built in just 16 months for 83 million dollars and completed on 15 January 1943, it has five sides of 281 m each, five floors and 28 km of corridors, yet no two points are more than seven minutes' walk apart. About 24,000 people work here, with their own food court, gym, chapel and six zip codes. On 11 September 2001, exactly 60 years after construction began, American Airlines Flight 77 was flown into the west side, killing 125 people in the building and 59 on the plane.

The Pentagon Memorial, dedicated on 11 September 2008, stands directly outside the impact site: 184 cantilevered benches, one for each victim, each over a small pool of water, arranged by year of birth and lit at night. It is the only place on the grounds where photography is permitted and the only part open to the public without a pre-booked tour.

Practical: The memorial is free and open 24 hours. Parking is limited; follow signs for the Pentagon Memorial from Route 27 / Washington Boulevard and use the designated visitor spaces. Building tours must be booked weeks in advance and are not needed to see the memorial. Do not photograph the building itself from the grounds; security will intervene.