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
Carbon-neutral fuel can be captured and buried, making fossil fuels carbon-neutral, although not renewable. Carbon capture from exhaust gas can make carbon-neutral fuels carbon negative. Other hydrocarbons can be broken down to produce hydrogen and carbon dioxide which could then be stored while the hydrogen is used for energy or fuel, which would also be carbon-neutral. The most energy-efficient fuel to produce is hydrogen gas, which can be used in hydrogen fuel cell vehicles, and which requires the fewest process steps to produce. Methanol can be made from a chemical reaction of a carbon-dioxide molecule with three hydrogen molecules to produce methanol