40d:Miner

Miners are necessary to build any kind of fortress, unless you're planning on building everything out of wood and on the surface.

Skill level increases the speed of mining and also increases the chance of leaving behind stone, ore or gems depending on the material mined. In the latter case, high skill can be actually undesirable, since excess stone can be a nuisance; however, miners gain skill extremely quickly so some amount of stone is unavoidable. Miners are required to dig, build channels, dig out stairs and ramps, and remove said stairs and ramps. Miners require a pick to do any work.

In order to start digging you need to designate an are to be dug. This is done with the Designations menu.

The mining skill is unique in that it is the only civilian skill that can be used in combat. Because miners quickly improve their mining skill during the normal course of mining, and because they always carry their picks with them, miners are extremely useful for defense in the early stages of the game. Woodcutters also carry their axes with them at all times, but axes use a special combat skill that is not practiced while doing civilian work. Miners will use their picks as weapons if they are drafted while they are carrying picks, and they are not designated to use any other weapon (i.e., they are instructed to fight unarmed).

Picks actually make rather poor weapons compared to axes, swords, spears and the like. However, skill matters much more than weapon quality in actual combat.

The material (copper, bronze, iron, etc) of a pick does not affect mining speed or quality, but it does affect combat strength (as it does for any weapon).