Mining

Underground mining involves the excavation of tunnels and rooms beneath Earth's surface. Compared to surface mining, underground mining is expensive and dangerous. Therefore, it is used primarily in situations where high-value ores such as gold are concentrated in narrow veins or other unusually rich deposits. Unlike surface mines, underground mines can also be excavated beneath bodies of water. Salt mines more than 328 yd (300 m) deep, for example, extend beneath Lake Erie near Cleveland, Ohio, and Detroit, Michigan.

The vocabulary of underground mining has developed over several centuries. Shafts are vertical passages excavated downward from Earth's surface, whereas raises and winzes are vertical passages excavated upward and downward, respectively, between horizontal workings beneath the surface. An adit is a horizontal passage excavated into a hillside, whereas an incline is a sloping passage excavated inward from a hillside. Horizontal underground passages following the trend of the ore body are known as drifts. An open room beneath the surface is a stope and its roof is known as the back.

Underground mines are excavated using a variety of methods. Room-and-pillar mining is the excavation of large open rooms supported by pillars. Coal and rock salt (halite) are commonly mined using room-and-pillar methods. Longwall mining is a form of underground mining widely used in the coal industry. A coal seam is completely removed using specialized machines, leaving no support and allowing the overlying rock to slowly subside as the seam is mined. Open-stope mining, in contrast, consists of rooms without any supporting pillars. It is employed if the ore body is small or the rock is strong enough to withstand collapse into the stope. Sub-level caving and block caving involve the excavation of vertical chutes and horizontal passages beneath an ore body, which is then allowed to collapse into the openings under its own weight. Gloryhole mining is a term used to describe caving methods that result in the formation of a crater or depression on the surface above the mine.

Certain water-soluble minerals can be removed from the Earth by dissolving them with hot water piped into the ground under pressure. This practice is known as solution mining. The minerals dissolve in the hot water and then are carried to the surface. In the Frasch process, a system of pipes is sunk into a known deposit of sulfur at some depth under ground. Steam forced into one pipe melts the sulfur, which is then extracted in a liquid form through a second pipe.