The 104-story One World Trade Center tower at the World Trade Center site in New York is expected to be finished in early 2014, and the 72-story 4 World Trade Center should open this fall. The underground museum is set to open next spring. A slideshow and time-lapse video by NBC looks at work at the site over the years, providing details about the structures and an explanation of why certain materials were used to build 1 WTC.

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