Are you familiar with the survivors in stairwell B of WTC 1, it went down the middle of the tower all the way to the bottom on B-6. The lower part of that stairwell was not crushed by the upper part of the building. It does not exactly support your theory. Here is the story:
http://nymag.com/nymetro/news/sept11/2003/n_9189/
Here is a picture of the stairwell taken during the recovery work:
http://www.photolibrary.fema.gov/photodata/original/4060.jpg (647.8 KB)
And very large parts of the core remained standing for some seconds after the rest of the towers had collapsed:
Here and as you can see on this
page if you scroll down to the collapse of WTC1. (
Think twice before you click on any of the pictures, because they are huge files showing film grain, without much more viewable details)
Bombs in the basement does not make any logical sense, the preparations would have increased the risk of blowing the plan tremendously.
I would think that the recovery workers would have noticed steel columns suspiciously cut in the towers core on the basement levels, when they were digging out and cutting the steel in the basement levels for transport.