source
My source for the 70 ft, was in a book, but here's an online source:
http://www.pbs.org/wgbh/buildingbig/wonder/structure/world_trade.html
The towers were 1365' tall. To dig 911' down would be to dig the equivalent of the 3/4 of the original towers down. A grand thought, but hardly practical in any conditions, let alone bedrock.
-Steve G.