S1, E2: The World Trade Center towers were once the tallest buildings in the world, marvels of engineering and architecture. But on 9/11, within two hours of being hit by a terrorist attack, they collapsed. The official report has left many questions unanswered and spawned countless conspiracy theories. But two scientists, working independently, have come up with the same new theory, based on missing evidence, which may finally explain what happened on that tragic morning.