The headquarters of the United States Department of Defense is the largest low-rise office building in the world. Approximately 24,000 military and civilian workers arrive daily. Astonishingly, the mammoth structure, completed in 1943, took less than two years to construct.
Following the September 2001 crash of hijacked American Airlines Flight 77 into the northwest side of the building, the damaged area was removed in just over a month and repaired in a year. In this same area is the America's Heroes Memorial and Chapel, which pays tribute to the civilians and military members killed in the attack. In 2008, the two-acre Pentagon Memorial, with its 184 benches commemorating the lives lost in 2001, was dedicated. Tours of the Pentagon are free; reserve online through the Pentagon Tour Office at least two weeks, but no more than three months, in advance.