The gas that fuels your cars actually comes from the remains of living organisms and is referred to as fossil fuels. This process, better known as anaerobic digestion, occurs when microorganisms break down biodegradable material in the absence of oxygen. The age of the organisms and their resulting fossil fuels are typically millions of years, and sometimes exceeds 650 million years. Although fossil fuels are continually being formed, they are generally considered to be non-renewable resources because they take millions of years to form.