Even given its size and complexity, the World Trade Center has taken an unusually long time to rebuild. If everything goes according to plan, the site won’t be finished until 2016. That’s nearly 8 years longer than the initial projections offered by New York’s then governor George Pataki in 2003. To give you an idea of how long that is, the original towers were completed in just five and a half years.

“It’s easy to ask, ‘What’s taking so long?'” says Chris Ward, the executive director of the Port Authority of New York and New Jersey, “but it’s harder to say, ‘O.K., this is how we build it.'” The World Trade Center construction site is a $20 billion venture — according to Ward, it is the biggest public-construction project that has ever been undertaken in the U.S. It is a vastly complex partnership between the Port Authority, a bistate government organization that oversees the regional transportation between New York and New Jersey; a private real estate developer named Larry Silverstein; and dozens of smaller companies and organizations that have been brought on to help design, build, fund and oversee everything from the subway and commuter-train center to a performing-arts venue. The site has suffered repeated delays, budget overruns, design changes and several serious lawsuits. After 9/11, it took nearly a year and a half for the city to even decide upon a rebuilding plan…

Today, the World Trade Center is the site of continuous, frenzied activity. To stay on schedule, the construction must go on 24 hours a day, stopping only during bad weather and, as was the case in May, when President Obama visits. “It’s extremely hard and grueling,” says Brian Lyons, a superintendent with Tishman Construction, the company that is building several of the Port Authority’s projects, including One World Trade Center. “You’re either climbing deep down into the ground or way up into the sky. There are so many different layers it’s absolutely mind-boggling.”

Like many of the 3,200 workers on site at any one time, Lyons works six or seven days a week, often for more than 12 hours at a stretch. He’s helped construct Seven World Trade Center, one of Silverstein’s buildings, successfully completed in 2006; One World Trade Center; and several other projects. He is currently overseeing the construction of the transportation hub. “We’re blasting, chopping and drilling. Meanwhile, there’s a live subway that passes us every four minutes during the day,” he says. “Because of that, most of the work is done in the middle of the night.”