1.   As the second of the towers toppled, a firefighter frantically backpeddled away from the debris cloud.

2.   In a third interview, Chief John Peruggia described being saved by a firefighter as he tried to outrace the debris cloud after the second collapse.

3.   Scientists have had a hard time explaining how any global darkness from debris clouds after such an impact could have lasted long enough to cause the widespread extinctions.

4.   The debris cloud was so thick that he had to use the handrails to guide himself over the familiar route.