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
Environmental impact of the coal industry to form fine particles and ground-level ozone and are transported long distances, making it difficult for other states to achieve healthy levels of pollution control. The wet cooling towers used in coal-fired power stations, etc. emit drift and fog which are also an environmental concern. The drift contains Respirable suspended particulate matter. In case of cooling towers with sea water makeup, sodium salts are deposited on nearby lands which would convert the land into alkali soil, reducing the fertility of vegetative lands and also cause corrosion of nearby structures. Fires sometimes occur in coal beds underground. When coal beds are