Here is a link to a very moving and interesting PBS/Nova video about the attack, the Memorial and the building of One World Trade Center. Walter Koenig brought it to my attention, Click Here: http://www.pbs.org/wgbh/nova/tech/ground-supertower.html

One World Trade Center...is the primary building of the new World Trade Center complex in New York City's Lower Manhattan and is the tallest building in the United States and Western Hemisphere. The 104-story supertall skyscraper stands on the northwest corner of the 16-acre (6.5 ha) World Trade Center site...located just south of One World Trade Center, where the Twin Towers once stood. The construction is part of an effort to memorialize and rebuild following the destruction of the original World Trade Center complex during the attacks of September 11, 2001. Wikipedia

The dark line up the center of the building is a construction elevator with clear floors. Here is a link to some image taken from inside the elevator.

Click Here: http://untappedcities.com/2013/12/17/daily-what-the-exterior-construction-elevator-at-1-wtc-has-a-see-through-floor/