I've noticed that most big explosions in Minecraft produce a rather sloppy hole, spread over a huge area, but not necessarily where you wanted it. This is a 70 x 70 hole from the surface to bedrock, pretty clean by TNT standards. Shafts of TNT dropped 7 blocks apart in a 10 x 10 grid. Precise
This method would produce a nice starting point for building a foundation, without compromising the surrounding area.