Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Coal mining year. The United States was ranked as the second highest coal producing country in the world in 2010, and possesses the largest coal reserves in the world. In 2008 then-President George W. Bush stated that coal was the most reliable source of electricity. However, in 2011 President Barack Obama said that the US should rely more on "clean" sources of energy that emit lower or no carbon dioxide pollution. For a time, while domestic coal consumption for electric power was being displaced by natural gas, exports were increasing. US net coal exports increased ninefold from 2006 to 2012, peaked at