http://www.ukcoal.com/down-a-modern-mine
http://www.ohiodnr.com/geo_fact/geo_f15/tabid/7872/Default.aspx
http://www.britannica.com/EBchecked/topic-art/122975/113913/Schematic-diagram-of-an-underground-coal-mine-showing-surface-facilities#tab=active~checked%2Citems~checked
http://www.canadianidentity.com/wiki/index.php/Sifto_Salt_Mine
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