There are hard-rock underground mines, and there are soft-rock underground mines. Coal deposits, for instance, live in relatively soft sedimentary rock. Gold deposits live in igneous or metamorphic rock, which is relatively hard, as do diamonds, copper, silver, nickel and zinc [source: Great Mining].
Even within the hard-rock category, design and extraction methods vary, but almost all revolve around a few basic techniques:
Room and Pillar – For relatively flat ore deposits, with little change in elevation throughout, miners drill an access ramp to the deposit and remove ore in a pattern of holes (rooms) and roof supports (pillars). The rooms can be mined out using conventional charge-and-blast techniques or, more commonly now, with a machine called a continuous miner. The continuous miner drills into the rock until it forms an excavated room, perhaps 20 to 30 feet (6 to 9 meters), leaving in place a rock pillar to support the "roof" [source: United Mine Workers of America]. The machine moves through the ore, creating rooms and pillars, until the full deposit is covered. A final pass drills through the pillars to recover the ore there, allowing the roofs to collapse behind the machine as it leaves each room.
Cut and Fill – For relatively narrow ore deposits, miners drill an access ramp adjacent to the ore deposit, from the surface down to the lowest point of the deposit. An operator then drives a drill through the ore, creating a drift, or a horizontal cut, from one side of the deposit to the other. In the hardest rock, no roof-support is needed; in softer rock, bolts may be placed in the roof as the drill progresses [source: Mining Know-How]. Once the drift is complete, backfill, or waste material, is spread into the open drift, creating a platform for the next pass. The drill drives on top of this backfill to cut another drift through the ore. This continues until the drill cuts a drift across the top of the ore deposit.
This method can be used in wider deposits, as well, by drilling two adjacent access ramps and cutting two adjacent drifts, often called drift and fill [sources: Mining Know-How].
Cut and fill is for hard rock, as it doesn't feature the support mechanisms inherent in and central to a method like room and pillar. The room-and-pillar approach, on the other hand, crosses easily into the softer stuff – and most coal mines.