Digging

Digging is usually done with a pick-axe (or a dwarvish mattock, appears to be a broad pick) or a wand of digging. You can also dig if you polymorph into a rock mole or woodchuck. You can dig in any direction, including downwards. If you dig upwards, you will just loosen a rock from the ceiling and have it fall on your head. Casting the dig-spell is same as zapping a wand of digging.

If you dig downwards with a pick-axe, you will create a pit. If you dig downwards in a pit, you will create a hole and fall through. Zapping a wand of digging downwards always instantly creates a hole.

The ground or the walls in some levels are undiggable. These include the walls of sokoban, the ground at the bottom of several dungeon branches, walls of the castle and the Wizard's Tower, and the like. The walls of Gehennom are diggable, but the wand and the spell only dig one wall per casting.