For most Americans, the image of the collapse of the World Trade Center towers was not only a scene of unforgettable horror, it was a moment of unimaginable consequence. Who could have guessed that a steel behemoth of such size and strength -- a building so massive that it had its own zip code -- could actually be reduced to 150 feet of dust and rubble? On "Why the Towers Fell," NOVA follows a blue-ribbon team of forensic engineers and presents their conclusions in the most definitive explanation yet seen by the American public of how and why the towers collapsed. .