The site has famously been a source of contention for everyone from politicians to developers, but it was many of the mourners who fought against rebuilding on what they called a sacred place.
"I think that's why it was so important that those people who felt immediately and understandably -- do nothing, leave this as a sacred site -- were wrong," Childs said. "It was an honor of the people that died there, they were working there, to rebuild and say, 'Pick yourself up.' It's in our DNA to want to rebuild after disasters."
Progress has been slow since the buildings fell amid a series of political and bureaucratic wrangling. Now, the work never stops. Construction has been nearly ongoing year round.
"It never stops," Baroni said. "We can't afford to stop."
Right now, there are 3,500 construction workers on all four corners of the site. By the time the project ends, Baroni said, 25,000 different construction workers will have worked on it.
Baroni said the schedule is set by a series of deadlines -- and the next one is for the museum -- set to open in 2012. Soon the office building will be finished, then the transportation hub, on which construction has yet to start.
"If you have to sum it all up in one word: progress," Baroni said. "Not a bad word, for a site that no one thought progress would ever happen."
ABC News' Dan Harris and Julia Bain contributed to this report.