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 pollution mitigation industry. Complaints focus on the environmental impacts of coal extraction, high costs to sequester carbon, and uncertainty of how to manage end result pollutants and radionuclides. In reference to sequestration of carbon, concerns exist about whether geologic storage of CO in reservoirs, aquifers, "etc.", is indefinite/permanent. The palaeontologist and influential environmental activist Tim Flannery made the assertion that the concept of clean coal might not be viable for all geographical locations. Critics also believe that the continuing construction of coal-powered plants (whether or not they use carbon sequestration techniques) encourages unsustainable mining practices for coal, which can strip away mountains,