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 in the United States foster positive public opinion of coal, many large coal mining companies, electric utilities, and railroads in the U.S. launched a high-profile marketing campaign to convince the American public that coal-fired power can be environmentally sustainable, despite the fact that coal is the largest contributor of CO2 emissions in the electricity sector. However, some environmentalists condemned this campaign as a "greenwashing" attempt to use environmentalist rhetoric to disguise what they call "the inherently environmentally unsustainable nature of coal-fired power generation". Coal mining has historically been a male-dominated industry. Today, Grassroots organizations opposed to coal mining are mostly led by women. When