Coal, salt, uranium, phosphate and oil shale live in soft rock and there are two primary methods in soft-rock mining: longwall and room and pillar [source: Great Mining]. Most coal is extracted using the room-and-pillar approach described previously [source: United Mine Workers of America]. However, longwall mining is exploding in popularity [source: Great Mining].
Longwall mining is extraordinarily efficient. Rather than drilling through the ore deposit, a longwall machine cuts across it, shaving off slices up to 600 feet (182 meters) long. Those slices drop directly onto a continuously moving conveyor, which carries it to a haulage shaft that lifts it out of the mine. In longwall mining, the roof supports are built into the machine, sitting between the top of the longwall miner and the roof of the room. As the machine progresses into the ore, the supports move with it, allowing the area behind it to collapse and fill in the excavated area.
The longwall method can recover up to 90 percent of the available ore. The room-and-pillar approach typically recovers about 50 percent [source: Illinois Coal Association].
When the ore deposit in relatively narrow, shorter cuts are made. This variation is called shortwall mining.
The old-school technique of blast mining, that uses explosives like TNT to break up ore, is still in use, but just barely – less than 5 percent of U.S. production [source: Great Mining].
Underground mining, then, is becoming increasingly efficient. It requires less human labor, which puts fewer miners in danger as they work deep beneath the surface. Still, underground mines are not the safest places to be.