An open mine, especially a coal mine, whose seams or outcrops run close to ground level and are exposed by the removal of overlying soil and rock. Strip mining can extract coal covered by as much as 60 m (200 ft) of rock and soil. If the seams roll through hills instead of lying flat, a series of tiers or contours are used to extract the coal or ore. Strip mining has been criticized for being ecologically destructive and for causing pollution of water resources, as the removed soil and rock are often dumped in lower-lying areas. Mining operators are sometimes required to restore soil and vegetation and to clean up the mining site.