I tried making a tunnel by making a 2x1 tunnel first and then lining it with a 1x1 line of TNT. Yeah... it doesn't work too well. You can check it out, its called splosion tunnel, I made some signs to it.
Also, I did a 256+ underground explosion by monroeski's tower (lololol sweet dreams monroeski), it did not breach the surface but it is CRAZY down there. You can find it by the huge dirt wall by monroeski's tower. The entrance is lit by two torches.

