A source of energy that comes from ancient, decomposed organisms is a fossil fuel. Natural gas and coal are both examples of fossil fuels.
Today's fossil fuels began forming millions of years ago, before dinosaurs lived on the earth. Over time, these plants and other organisms died, decomposed, and sank to the bottom of the ocean. Over time, heat and pressure eventually turned them into coal, oil, and gas. While the world depends on fossil fuels to power vehicles and produce electricity, they are ultimately very hard on the planet, creating enormous amounts of carbon dioxide.