All three types of mines cause some damage to the environment. Surface mines are the most damaging to the environment, because they cause the greatest amount of change to the land. Surface mines involve the removal of earth to extract minerals, often leaving a gaping hole behind. Subsurface mines have less permanent damage to the environment, although they still produce many byproducts that are introduced into the environment. Placer mining can be less damaging to the environment than surface mines, as the sediment is returned to the water after the minerals have been extracted.