I guess everybody still knowns where he or she was when terrorists transformed a part of Manhattan into Ground Zero on 11th of September, 2001 (I myself was doing my civilian service in Göttingen, Germany). This date has eternalized in the memories of people around the world. You can now visit the memorial and a very impressive museum at the original location.
The memorial shows the boundaries of the building that was destroyed by airplane attacks. Water is falling down into the void, the names of the victims engraved around. If you go down into the museum you see photographs that remind you on the views on this building, that you have seen so many times and where so many tourists have taken pictures on.
You walk on different levels around the original, subterranean structures of the building. Exhibited are memories of these days, pictures that went around the world and video installations of mass media reflecting what happened. What makes this museum really impressive is, that the remains are preserved and you really can experience what happened.
You can get to the museum via subway going to West street or Cortland street. You should by your tickets online, get a time slot and calculate some time for a security check.
180 Greenwich Street
New York, NY