The debris pile in the footprint of WTC1 is absurdly small -- it is 20 feet deep at most. We're talking about a 1360 foot building turning into a 20 foot pile of rubble.
Excluding the core and structural core, if you simply use a very conservative figure of 6 inches of solid compacted material per floor, the fallen floors should have been AT MINIMUM 55 feet of debris.
IN REALITY the debris pile would have been much bigger than 55 feet in height due to the huge core columns that came down along with the floors.
Even if you assume the debris was spread over twice the footprint of WTC1 (though remember, we are talking about the FLOORS here, not the walls that clearly blew outwards-- the floors appeared to have gone straight down)-- we are talking a debris pile of 27 feet AT MINIMUM, just from the floors.
Now-- it is possible that the basement region of WTC1 collapsed-- though I can't find any evidence for that and this clearly didn't happen for WTC2-- and that much of the WTC1 debris went in there. It seems to me that if the basement region collapsed, we would see more of a cratering effect around WTC1, and this clearly is not seen. Nonetheless, the possibility that much debris went below the ground is why this debris pile is merely very suspicious rather than an absolute smoking gun.