The National September 11th Museum has a nearly impossible task: to tell the definitive story of that devastating Tuesday morning 10 years ago. For visitors next year and a hundred years from now, this will be the place to go to understand what happened. In order for one museum to shoulder such a massive responsibility, its builders and curators will have to meet some incredible challenges.

The first challenges are both logistical and technological. The plan calls for an above-ground entrance pavilion in Memorial Plaza, which sounds pretty straightforward. But from there visitors will descend below the plaza into a vast underground exhibit, down into a 100,000-square-foot museum space. The museum proper will be built underneath half of the entire 16-acre World Trade Center site, alongside the Path train, beneath one of the world's largest fountains. It will not just be constructed underground - which is hard enough - but be completely enmeshed in one of the largest and most complicated construction projects in history.

The degree of construction difficulty here is unprecedented, but it is necessary. Only underground can visitors experience the full measure of what the towers were and what happened to them. Underground, visitors will be able to see the foundations of the original World Trade Center and even part of the massive slurry wall that holds back the force of the Hudson River, a wall that just barely held up as the towers fell.

And only underground can the museum claim the vast space it needs to explore its subject fully. Some of the 9/11 artifacts the museum will display are enormous: crushed firetrucks and taxicabs, the impact steel that took the hit from one of the planes, even the staircase survivors used to flee the North Tower. They require space. They also require ingenuity to transport them, carefully, sometimes in pieces, from their current home in a hangar at JFK airport.

These engineering challenges, great as they are, will be rivaled by the challenges the curators face in telling the 9/11 story. When someone says "Sept. 11" or "World Trade Center," it conjures up so much: the horrific attacks of that day, the concentrated hatred behind them, the deaths of almost 3,000 people, the rubble at the heart of the city, the worldwide outpouring of support, the "post-9/11" shift in U.S. policy and self-image, the geopolitical tremors that shook the world. The museum has not one story to tell but a dozen, a hundred, even thousands, as it seeks to do justice not just to grand historical and political themes but also to the intensely personal experience of the victims. Visitors will be awed by the two giant tridents that formed the base of the fallen towers, but they will also be moved by handmade signs of families searching for loved ones in the aftermath.

Half of those loved ones were never found; they disappeared on that day. But it's still possible that they didn't disappear without a trace, and one part of the museum will actually contain a working DNA lab whose mission is to give closure to the families of more than 1,200 victims who never had remains to bury.