What fuels were formed from the decayed remains of plants and animals?

The fuels formed from the decayed remains of plants and animals are fossil fuels. The main fossil fuels are coal, petroleum and natural gas.

They formed over millions of years as organic matter accumulated in environments such as ancient swamps, lake beds and marine sediments. Burial beneath layers of sediment subjected the remains to heat and pressure, gradually transforming them into carbon-rich fuels. Coal developed mainly from buried plant material, while petroleum and natural gas formed largely from microscopic marine organisms and other organic matter.

The word "fossil" refers to their ancient biological origin, not to visible fossils inside the fuel. Fossil fuels are nonrenewable on a human timescale because they take vastly longer to form than people use them. Burning them releases energy, but it also produces carbon dioxide and other pollutants.

Fossil fuels are often confused with biofuels, which also come from biological material but are produced from relatively recent crops, plant matter or waste. Nuclear fuel is another separate category: it releases energy through nuclear reactions rather than by burning ancient organic remains.