Longwall mining is a highly productive underground coal mining technique. Longwall mining machines consist of multiple coal shearers mounted on a series of self-advancing hydraulic ceiling supports. The entire process is mechanized. Longwall mining machines are about 800 feet (240 meters) in width and 5 to 10 feet (1.5 to 3 meters) tall. The miners extract "panels" -- rectangular blocks of coal as wide as the mining machinery and as long as 12,000 feet (3,650 meters). Massive shearers cut coal from a wall face, which falls onto a conveyor belt for removal. As the miner advances along a panel, the roof behind the miner's path is allowed to collapse.
Longwall mining was first introduced in the 1950s and 1960s. Today it accounts for more than half of all coal production in the United States. On any given day, a typical longwall mining system is capable of extracting between 10,000 and 30,000 tons (9 to 27 million kilograms) of coal from a panel. The primary downside to this very productive technique is a prohibitive initial investment -- these mining machines usually run between 5 and 15 million dollars.
Longwall mining replaces the historical "room-and-pillar method", whereby underground "rooms" of coal are manually extracted and pillars are left to support the roof so miners can work safely. In mining regions deeper than 1,000 feet (300 meters), the room-and-pillar method becomes highly uneconomical because the size of pillars required to support the roof are much larger, meaning that valuable coal cannot be extracted from them. Longwall systems make deep mining feasible.
Sometimes this type of mining is called destructive or environmentally unsafe because it causes the land above the mined-out panel to sink. This can damage underground water tables, structures at the surface, and can cause erosion of the soil. Careful geological surveying can help ameliorate these problems. As technological advances continue to make longwall miners increasingly effective, they will become responsible for an increasing portion of the world's total coal production.