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This drawing depicts the three types of underground mines--shaft mines, slope mines and drift mines. The decision of what type of mine to construct depends on the depth of the coal seam and the surrounding terrain.
Drift mines have horizontal entries into the coal seam from a hillside. Slope mines, which usually are not very deep, are inclined from the surface to the coal seam. Shaft mines, generally the deepest mines, have vertical access to the coal seam via elevators that carry workers and equipment into the mine.
Almost all underground mines are less than 1,000 feet deep, but some mines reach depths of about 2,000 feet. UMWA miners in Nova Scotia actually mine coal beneath the Atlantic Ocean.
