ARLINGTON, Va. -- The traumatic events of Sept. 11, 2001, changed the lives of everyone in America, especially those working for the Defense Department. Terrorists proved that the very symbol of our national security -- the Pentagon -- was not, in fact, impenetrable.A hijacker flew American Airlines Flight 77 into the southwest corner of the Pentagon that day, killing 184 people -- all on board the plane and many more inside the building, which was heavily damaged.The Pentagon has since rebuilt, and an important addition to the grounds is the National 9/11 Pentagon Memorial honoring those who were lost. It was dedicated on Sept. 11, 2008 -- seven years to the day after the attacks.THE MEANING BEHIND THE DESIGNThe $22 million memorial sits on two acres of land right outside where the jetliner struck. Dozens of crape myrtle trees surround the serene landscape. There are 184 memorial benches dedicated to each of the victims, and they're organized in a timeline of their ages, from the youngest victim, 3-year-old Dana Falkenberg, to the oldest, 71-year-old John Yamnicky.Each bench contains a pool of water that reflects light in the evenings, and they're all positioned in a way that distinguishes the victims onboard the airplane from those inside the Pentagon. The benches for the 59 jetliner passengers are positioned so a visitor will face the sky when reading the victim's name. Those dedicated to the victims inside the building have the victim's name and the Pentagon in the same view.The site also has a curved wall, aptly dubbed the Age Wall, which gets higher in height to represent the ages of the victims, meaning it starts at 3 inches tall to represent Falkenberg and grows to 71 inches to represent Yamnicky.ARE THERE TOURS OF THE MEMORIAL?While there are no official guided tours, there are guides on the grounds to give out information, and there is a 24-minute audio tour that's available for download. You can also call 202-741-1004 at the memorial entrance and listen to the tour that way. The audio provides a sequential narrative of the events at the Pentagon on Sept. 11, 2001. It also explains the purpose of the memorial's design and the building's history.THERE'S A MEMORIAL CHAPEL INSIDE, TOOThe Pentagon Memorial Chapel was built within a year and dedicated on the first anniversary of the attacks. The point of impact of Flight 77 had become a place where the Pentagon community left flowers and mourned the victims, so senior leaders decided that instead of rebuilding that area as office space, it would become a chapel for all faiths to worship and pray.The chapel is open 24 hours a day and can support 12 worship services a week. It's conveniently located right next to the Pentagon Chaplain's Office.In the front of the chapel, a memorial window was designed to look like the Survivor's Pin, which was given to every person who lived through the Pentagon attacks. It includes 184 pieces of red glass representing those who died.Four other stained glass windows in the chapel are dedicated to the attack victims. One is dedicated to the Navy, which lost 33 sailors and nine civilians that day. Two other windows are dedicated to Army departments that lost a total of 22 soldiers and 53 civilians. The fourth window is dedicated to the Defense Intelligence Agency, which lost seven employees that day.The chapel is consistently used and is even a permanent part of the Pentagon tour.A VISITOR'S CENTER IS IN THE WORKSThe Pentagon Memorial is currently the only 9/11 attack site that doesn't have a visitor's center or museum. That's going to change now that many visitors, especially children born after the tragedy, don't fully understand its significance.The 9/11 Pentagon Memorial Visitor Education Center is expected to be built to the west of the memorial itself, on the path of Flight 77 before it hit the building. The project is expected to be complete by early 2020. Construction is expected to take two years.WHAT TO KNOW IF YOU'RE VISITINGYou might have a lot of questions, like how to get there. Parking is strictly enforced at the Pentagon, so your best bet would be to take public transportation; however, handicapped parking is available.Food, drink and pets aren't allowed at the memorial, but you can take a memento to leave behind. Smoking is not allowed. Photographs can be taken at the memorial, but not anywhere else on the Pentagon Reservation.