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Photo: via Wikimedia Commons, Public domain
Just outside the Pentagon, and accessible to visitors
The National 9/11 Pentagon Memorial, or simply the Pentagon Memorial, located just southwest of the Pentagon in Arlington County, Virginia, across the Potomac River from Washington, D.C., is a permanent outdoor memorial to the 184 people who died as victims in the building and on American Airlines Flight 77 during the September 11 attacks.
© OpenStreetMap contributors
While lingering is not recommended for security reasons, you should know it is the largest office building in the world, and covers 4 zip codes. (Army, Navy, Air Force and Department of Defense). Tours are available by online reservation made 14-90 days in advance. The Pentagon Memorial is open 24 hours to visitors on the Washington Blvd side, where Flight 77 hit. Photography is allowed at the memorial, but is not permitted anywhere else on the Pentagon grounds— take photos anywhere else on site, and you may face a lengthy interrogation by the Pentagon Police and will probably be asked to delete the images. On a lighter note, the interior courtyard is irreverently referred to by employees as "Ground Zero," as it was the target of a number of Soviet missiles during the Cold War. There is no public parking at the Pentagon but it is about a 5-minute walk from the Pentagon City Mall through a tunnel.
A monument dedicated in Fall 2006 to the U.S. Air Force. The monument consists of three soaring spires signifying the Thunderbird's bomb-burst formation. Also on the site are a few statues and information about various airborne campaigns the U.S. Air Force has participated in. The monument is on a hilltop with an excellent view of Arlington Cemetery, the Pentagon, and Washington DC. Limited parking is available in the lot across the street from the memorial.
On Columbia Island, along the Mount Vernon Trail
The memorial honors the contribution of women to America's military. In 1932, McKim, Mead and White designed a gateway to Arlington Cemetery, and in 1997 a mostly underground museum opened behind that historic structure. It features informational displays and historic artifacts and uniforms. A national parks passport stamp is available at the gift shop's checkout.
Just as the name says, this tomb sited on a picturesque hill overlooking Washington, D.C., is the resting place of one soldier each from World War I, World War II, and the Korean War whose bodies could not be identified. (Due to advances in DNA testing, a soldier from the Vietnam War formerly entombed here was identified and reinterred, and it's likely that there will be no more unidentified soldiers from now on.) Guards in pristine dress uniforms maintain a 24/7 patrol in a meticulous back-and-forth pattern: 21 steps, turn and shoulder arms, pause for 21 seconds, and repeat. Even inclement weather does not stop them (although there is a shelter used very occasionally in extreme conditions). The prestigious post is not entirely ceremonial; the guards are armed and will interrupt their routine to silence loud or disrespectful visitors or stop anyone (including children and tourists with cameras) who approaches too close. A formal Changing of the Guard ceremony is held half-hourly in summer and hourly in winter. The solemn ceremony is a popular sight to watch, but if you want to spend some time in quiet contemplation you may find the crowd and their cameras disruptive; instead, aim to arrive just after the ceremony when the crowd thins out until the next one.