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Coal is a fossil fuel formed in ecosystems where plant remains were preserved by water and mud from oxidization and biodegradaton, thus sequestering atmospheric carbon. Coal is a readily combustible black or brownish-black coal. It is a sedimentary rock, but the harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. It is composed primarily of carbon and hydrogen along with small quantities of other elements, notably sulfur. Coal is extracted from the ground by coal mining, either underground mining or open pit mining