A memorial at the entrance to Massey Energy's Upper Big Branch coal mine represents the 29 coal miners who were killed in an explosion in 2010. Jeff Gentner/AP hide caption

itoggle caption Jeff Gentner/AP

The city of Gladstone near the Great Barrier Reef is the world's fourth largest coal-export hub. Dredges, like one seen here, have turned the harbor brown as they work to expand the coal port. Richard Harris/NPR hide caption

itoggle caption Richard Harris/NPR

China and India are projected to propel coal's challenge of oil as the world's top energy source within the next five years, according to a new study. Here, a man rides a bicycle toward a coal-fired power station in China's Guangdong province last year. Mark Ralston/AFP/Getty Images hide caption

itoggle caption Mark Ralston/AFP/Getty Images

Wreaths and flowers commemorating those killed in the Crandall Canyon Mine collapse are posted at the mine entrance on Dec. 10, 2007, outside Huntington, Utah. A settlement Thursday includes mine owner Murray Energy's acceptance of four violations considered "contributory" to two fatal August 2007 mine collapses at Crandall Canyon. Douglas C. Pizac/AP hide caption

itoggle caption Douglas C. Pizac/AP

As U.S. coal consumption has fallen, its exports of coal have risen. Pictured, Midwest Generation's Crawford Generating Station, a coal-fired power plant in Chicago. The city's two coal-fired plants are closing under a deal with city officials and environmental groups. M. Spencer Green/AP hide caption

itoggle caption M. Spencer Green/AP