FBI agents, firefighters, rescue workers and engineers work at the Pentagon crash site on September 14, 2001, after a hijacked American Airlines flight slammed into the building on September 11.
Take a look, if you can stand it, at video footage of the World Trade Center collapsing. Your eye will naturally jump to the top of the screen, where huge fountains of dark debris erupt out of the falling towers. But fight your natural instincts. Look farther down, at the stories that haven't collapsed yet.
In almost every clip you'll see little puffs of dust spurting out from the sides of the towers. There are two competing explanations for these puffs of dust: 1) the force of the collapsing upper floors raised the air pressure in the lower ones so dramatically that...
