[hinkley]

When you core a tree to look at growth rings you have to be fast because the wood will begin to swell almost immediately. If you get distracted then you lose the corer. The tree is stronger than the handle.

I would think rocks under high pressure would do the same thing. You’d either have to dig a conical hole to get a cylindrical bore, or the boring machine would have to have cutters on both the face and the sides to keep grinding away rock that expands into the void you are creating.