The structure, which took 13 years to create, commemorates not only the 3,000 who died on September 11, but also the victims of the ’93 bombing, which killed six. At the entrance of the museum there are clusters of 400 white oak swamp trees surrounding two large recessed pools where the towers once stood. Water falls from the rims of the acre-wide pools where the names of victims, including those from the Pentagon, American Airlines flight 77, United Airlines flight 93 and from the 1993 bombing, are inscribed. (These are the largest man-made waterfalls in the United States.) To enter the structure, visitors descend 30 feet below street level to view various artifacts including American Airlines slippers found at the site, fire trucks and steel beams twisted by direct impact with the planes and to learn about the facts leading up to the day and those following. Take the NYT virtual tour here.