Fuels made from long dead lifeforms.
Coal was made from plants sinking into shallow water, then being buried underground.
Oil is made from plankton, microscopic plans and animals floating in the ocean. When they die they fall to the sea floor and are buried in sediment.
Gas is produced by any organic life form decomposing.
We can convert organic waste to oil by combining it with hot water under pressure. This may be an acceleration of the natural process.