Digging

Digging is usually done with a pick-axe (or a dwarvish mattock, appears to be broad a pick) or a wand of digging. You can also dig if you polymorph into a rock mole. 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.

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 the Gehennom are diggable, but the wand and the spell only digs one wall per casting.