Natural gas ?

Natural gas - like oil and coal - is a fossil fuel. It is a “nonrenewable” fuel because it takes millions of years to form deep in the earth before it can be used again. When microorganisms, sea plants and animals died over 300 million years ago (even before the dinosaurs!), they settled on the bottom of the oceans, which covered most of the earth. Layers of these dead sea plants, animals, sand, mud, and other debris built up over time and the pressure and heat from the earth turned them into petroleum and natural gas. The natural gas would get trapped in pockets of underground rocks. Sometimes the gas would seep up to the earth's surface and dissipate into the air or ignite from lightning strikes.
Natural gas in its pure form is colorless, odorless and shapeless. Only after processing is the "rotten egg" odor added for safety. Natural gas is actually a mixture of many gases, including propane, butane, carbon dioxide, ethane, oxygen, nitrogen and hydrogen sulfide, but it is primarily made up of methane gas. When you burn natural gas, it gives off heat energy that is stored in the gas molecules. There are some renewable sources for methane, like garbage in landfills, that produce some gas.