Various people have suggested that the basement of the WTC was blown with high explosives (even nuked) prior to, and/or concomitant with, the top-down demolition that was performed to obliterate the towers.
I have always been somewhat unsure of this idea for the simple reason that it wasn't clear to me how exactly basement bombs/nukes fit in with the top-down demolition-- plus there was the fact that during the demolitions, part of the inner cores for each tower remained standing after the initial demolition phase.
However, this post from 911blogger provides more evidence for explosions at the base of the tower concomitant with demolition.
Thinking about this some more, I have an idea for how basement bombs would have been used.
Here we see the remnants of tower one-- and clearly there is a section of the core remaining in the center of the blasted out debris:
The key here is that IN FACT, only PART of the core is remaining-- what looks like only the very center of the core-- perhaps a dozen columns at most.
The core was made of 47 absolutely MASSIVE columns-- just think that the core columns that would make up the core at the base of the tower carried much of the weight of those huge towers above. Clearly, most of these massive columns have been blown away from their original location. Note, it's not like there is debris is covering these columns-- the columns have either been knocked/blown away or they have been blown to pieces. Given that the very center core columns survived the "onslaught" (see the picture), it is not clear why the just as strong outer core columns wouldn't survive-- unless they were literally blown away at the base.
What seems plausible-- even likely-- is that several basement nukes took out the core columns at the sides or corners of the core section. A partial dismemberment of the core could conceivably facilitate a top-down collapse by taking away some of the supports between the floors and the core. This would give explosive devices on the upper floors an easier time in peeling the outer walls from the core remnant. Which is really what happened-- the outer walls peeled away from the tower from top to bottom, in an explosive sequence-- leaving an inner core remnant that then must have been blown from below from the lower floors to leave the small column remnants seen in the picture above.
But the key here is that basement bombs-- almost certainly nukes-- would be used to take out PART OF THE CORE specifically to facilitate the top-down collapse.