I was just scrubbing over some WTC7 collapse video when I noticed something new to me. When the East penthouse falls though the building it creates a visible wave on the windows, presumable as they are twisted or pushed out somehow by the changes in pressure. The scale of the pressure is also visible by the smoke being sucked in at the top of the building. Source: https://www.youtube.com/watch?v=Wf1HcJ7GKsc This is relevant because it appears to show the East Penthouse falling rapidly though the building, which is consistent with a collapse of C79/80/81 at a low level, well before the other columns failed. I think some people, like @Tony Szamboti, have suggested that the East Penthouse actually only fell in a few floors, and that lower interior column failures were simultaneous, but this seems at odds with that. You can kind of see this in the more familiar footage on the right, but the angle and lighting are not perfect, and there's more obscures. Contrast has been adjusted individually on all three videos.