Yet setting off half a tonne of TNT on the ground, or even just under it, won't penetrate 60 meters deep, or even 15; it will just blast open a shallow crater.

Yet setting off half a tonne of TNT on the ground, or even just under it, won't penetrate 60 meters deep, or even 15; it will just blast open a shallow crater. A shaped charge will do only a little better.