Three hundred million years ago, the climate of much of the Earth was mild and warm, and plants grew year round. Vast swamps were filled with plant species that have long since become extinct. Many of these plants—horsetails, ferns, and club mosses— were large trees.

    Plants in most environments decay rapidly after death, due to the activities of decomposers such as bacteria and fungi. As the ancient swamp plants died, either from old age or from storm dam­age, they fell into the swamp, where they were cov­ered by water. Their watery grave prevented the plants from decomposing much; wood-rotting fungi cannot act on plant material where oxygen is ab­sent, and anaerobic bacteria, which thrive in oxy­gen-deficient environments, don't decompose wood very rapidly. Over time, more and more dead plants piled up. As a result of periodic changes in sea level, layers of sediment (materials deposited by gravity) accumulated, covering the plant material. Aeons passed, and the heat and pressure that ac­companied burial converted the plant material into a carbon-rich rock called coal, and the layers of sediment into sedimentary rock. Much later, geo­logical upheavals raised these layers so that they were nearer the Earth's surface.

    Oil was formed when large numbers of micro­scopic aquatic organisms died and settled in the sediments. As these organisms accumulated, their decomposition depleted the small amount of oxy­gen that was present in the sediments. The resul­tant oxygen-deficient environment prevented fur­ther decomposition; over time, the dead remains were covered and buried deeper in the sediments. The heat and pressure caused by burial aided in the conversion of these remains to the mixture of hy­drocarbons (molecules containing carbon and hy­drogen) known as oil.