OSM Page:http://wiki.openstreetmap.org/wiki/Proposed_features/Surface_Mining
http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dmineshaft
http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dadit
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I've taken those 2 proposals/tags and made up for both over and underground mines.
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RP: looks great
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man_made=adit for horizontal entrances into hillsides.