Although the terrible loss of human life — 2,762 people dead, including all 156 passengers and crew on the four hijacked airplanes — will always remain the most tragic consequence of the events of September 11, 2001, the extraordinary physical destruction in New York City brought on by the terrorists has had enduring and complex implications of its own — as experts, policy makers, and ordinary citizens attempt to understand why and how the towers fell, and look to what should be built in their place.

Destructive Missile
When the first Boeing 767 tore into the north wall of the north tower of the World Trade Center at a speed of nearly 500 miles an hour (or more than 200 miles per hour faster than commercial aircraft are allowed to fly at low altitude), the enormous, 156-foot wide airplane was transformed into a 137-ton missile of aluminum and steel, destroying everything in its path.

Structural Miracle
Amazingly, the tower itself stood, absorbing the unimaginably powerful impact. The extraordinary robustness and redundancy of the building's structural system meant that although a giant gash now ran across much of the north wall — severing no fewer than 35 of that wall's 59 columns — the structure managed to redistribute the load to undamaged columns on either side of the opening, maintaining its integrity.

A Second Miracle
The same held true 16 minutes later, when the second plane slammed into the south tower, this time traveling nearly 600 miles per hour and thereby releasing, because of its higher speed, half again as much kinetic energy as the first attack. Just like its twin, despite the severing of 30 of the 59 columns along its south face, this tower continued to stand — as its engineers, who decades earlier had explored the possibility of an accidental impact by a jetliner, had determined it would. As before, the tower's thick fence of exterior columns, firmly linked by heavy steel "spandrels" on every floor — a design built to resist the worst hurricane winds imaginable — allowed the structure to efficiently redistribute the load to either side of the 70-foot wide gash, effectively forming an arch over the hideous opening. As a consequence of that structural miracle, nearly everyone in the two buildings below the point of impact — at least 10,000 people — escaped with their lives.

Lethal Fire
No one, however, had ever planned for the deliberate attack by a plane loaded with 9,000 gallons of highly inflammable aviation fuel. The spectacular fireballs that exploded outside of both buildings (as, in each case, about 3,000 gallons of fuel suddenly ignited) stunned everyone who saw them, but did little relatively structural damage to the buildings. It was the 6,000 gallons still remaining that proved lethal, pouring down stairways and elevator shafts and igniting everything in their path. Like all modern high-rises in New York, the World Trade Center boasted an extensive network of sprinklers, standpipes, and water hoses capable of putting out the worst fire that could ever be fed by the interior furnishings. But all of those systems had been destroyed or rendered useless when the wings and fuselages of the jet planes, ripping through the buildings, severed the pipes that supplied the sprinklers and hoses, disabled all of the elevators, and blocked nearly all of the staircases.

Vulnerable Elements
After nearly an hour of continuous burning, the temperature of the flames inside the towers had reached in excess of 1,500 degrees Fahrenheit. Those fires were now having a devastating effect on the buildings' structure — especially, many experts later concluded, on its most vulnerable elements: the steel trusses linking the columns of the outside wall to those at the center of the building. Located underneath each floor, the trusses served not only to carry the weight of the office floors but to provide crucial bracing for the building's outer columns, keeping them from buckling under the enormous load of the structure above. They were fireproofed with a sprayed-on coating of mineral fiber — but much of that covering had been jarred loose by the initial impact, leaving the open, weblike trusses exposed to the intense heat, which was steadily robbing the steel of its strength. As the trusses sagged, engineers later speculated, they began pulling away from their connections, fatally uncoupling the building's structural system.

"Pancaking"
Whatever the precise sequence of events within, at a few moments after 9:58am, along the east wall of the 80th floor, the building's exterior columns began buckling. Moments later, the top 30 stories of the south tower suddenly twisted to the east and south, and the entire building began to collapse, each floor slamming down on the floor below it and causing that floor, in turn, to fall onto the one below — a phenomenon known as "pancaking." From start to finish, the building's collapse took little more than 10 seconds. Thirty minutes later, the north tower collapsed as well, falling in about the same amount of time. Unlike the south tower, whose exterior columns had given way, the north tower's columns had collapsed from within, as its core of structural columns, weakened by the intense heat and the devastating damage caused by the impact, finally buckled and fell. It had stood for 102 minutes and five seconds.

Widespread Destruction
The collapse of the two towers had brought on the destruction of the five other buildings in the Trade Center complex, including Number 7, a 47-story tower that fell at 5:20pm, after burning for most of the day. In all, 12 million square feet of office space — a total as large as the entire downtown areas of many major American cities -- had been destroyed. Another nine million square feet had been rendered unusable without major repair and reconstruction.

Infrastructure Damage
Yet even that staggering total understated the overall damage to the city's physical infrastructure. The attack had destroyed or seriously damaged several major switching hubs for telephone and cellular lines, along with the giant antenna atop the north tower, leaving much of the city without phone service, broadcast television or radio. The collapse of 7 World Trade Center had destroyed a major electrical substation located beneath it, as well as the multi-million-dollar Emergency Command Center on its 23rd floor. And two mass transit lines — a spur of the IRT subway and the PATH commuter train that ran beneath the World Trade Center — had been destroyed when the towers came down.

Rapid Rebuilding
Within weeks, work began on the replacement or restoration of many of these essential systems. Within a year, the subway tunnel had been rebuilt, and service restored, while planning had begun on the reconstruction of the PATH line, not only to restore the original service, but also to provide a genuine improvement — a new connection to the city's subway system. It was the start of what planners intended to be a new downtown "Grand Central," a transportation hub that would not only link several existing subway lines and the PATH train but, it was hoped, new high-speed lines to Kennedy Airport and the Long Island Rail Road. This new center would fulfill a goal that had been talked about for decades but never acted upon — to provide lower Manhattan with the kind of regional transportation hub that midtown Manhattan had enjoyed since the start of the 20th century.

An Eternal Memorial
It was in that same spirit that planners began exploring the future of the World Trade Center itself, hoping to find new possibilities for the future of the city in the tragedy. Everyone recognized that a significant part of the site would need to be set aside as a memorial to the thousands who had been lost there. Indeed, as proposals for an international competition took shape, planners anticipated that the memorial would become one of the most heavily visited places in the world.

Respect for the City Grid
Much of the site was to be rebuilt as urban fabric, not only to replace the revenues generated by the trade center, but as a way of making Lower Manhattan whole again. By the start of 2002, officials had begun to put in place a process for re-envisioning the future of the 16-acre site. From the earliest proposals, it was obvious that the World Trade Center's 1960s "superblock" plan, which effectively turned its back on the rest of the city, would be superseded by an approach that, in concordance with the ideas of urban activist Jane Jacobs, respected the traditional grid of streets and blocks.

Liberty Tower
In the months to come, a convoluted planning process — by turns inspiring and frustrating — would proceed as state and city officials, civic groups, and private developers attempted to determine not only what should be built, but how to go about making the decision. The release of six massing studies for the site, developed by local planning consultants, provoked so vocal an outcry — especially from 5,000 New Yorkers gathered at an extraordinary daylong public forum at the Jacob Javits Center — that the project sponsors, rapidly changing gears, chose to cast their net more widely, inviting noted architects from around the world to submit proposals. That approach yielded nine initial designs, then two developed schemes, then a single striking proposal by the Polish-born architect Daniel Libeskind, featuring a 1,776 foot-tall "Liberty Tower" (along with several other office buildings), a series of lower-rise cultural institutions, and a large memorial located in the sunken pit defined by the original concrete "bathtub" which surrounded the towers' foundations. Though there were sure to be many changes in the months and years to come, the Libeskind scheme, officials hoped, would provide a basis for moving forward.