Subsurface mining is the removal of deposits from the earth by drilling underneath layers of rock and dirt. These deposits are difficult to reach from the surface and require the drilling of vertical or horizontal shafts for access. The shafts are reinforced with supports before workers access the tunnels.
How the subsurface minerals are mined depends upon the type and depth of the mineral. Solid subsurface resources are often blasted out of rock with explosives. The solid materials are gathered by workers before being brought up the mine shaft in carts or railroad cars. If the mined resource is a liquid, it may be pumped out of the shaft. Some water-soluble minerals can be flushed out with water and then recrystallized when brought to the surface.
Subsurface mines are generally less than 1,000 feet deep, but some reach as deep as 2,000 feet. The deepest resources often require the drilling of shaft mines. Shaft mines are vertical mines that require the use of elevators to get the product and the workers out of the mine. Drift mines enter the ground horizontal to the deposit. Slope mines are generally shallower and enter the ground through a hillside before sloping down to the deposit.