1. About coal, http://www.iea.org/topics/coal/.
2. World Coal Institute, The Coal Resource: A Comprehensive Overview of Coal. http://www.worldcoal.org/file_validate.php?file=coal_resource_overview_of_coal_report(03_06_2009).pdf
3. http://www.ucsusa.org/clean_energy/our-energy-choices/coal-and-other-fossil-fuels/how-coal-works.html#.VpUis-_SlMs.
4. Surface Mining;Kennedy,1990
5. U.S. Coal Consumption by End-use Sector;EIA,2007