Illustration showing the process of coal formation. The earth once had many dense forests in low-lying wetland areas (Fig. 1, left). In such environments, the accumulation of plant debris exceeds the rate of bacterial decay of the debris. The bacterial decay rate is reduced because the available oxygen in organic-rich water is completely used up by the decaying process. These conditions produce the dense, carbon-rich material known as peat (Fig. 2, middle) which, over millions of years, becomes compressed and heated, turning into coal (Fig. 3, right).