Natural gas is the cleanest of the fossil fuels.
It's composed almost entirely of methane, which is one carbon and four hydrogen atoms, or CH4. When it burns, it joins with oxygen to release water and carbon dioxide, and releases valuable energy. Most geologists believe it was formed from decomposed plant and animal matter that accumulated over millions of years. Layers of sand, dirt and mud covered the decomposed matter, and eventually changed to rock. The buried plants and animals decayed into tiny bubbles of colorless, odorless gas that's now trapped in the pores of the rock.