Natural gas began with tiny plants and animals that lived more than 200 million years ago—even before dinosaurs roamed the earth! Tiny water creatures and swamp plants died and were covered over by mud, sand, and silt. Over millions of years, heat and pressure inside the earth turned their decaying remains into fossils, and then into natural gas.

Like other fossil fuels, natural gas is found deep underground. A rock formation called a gas trap allows the natural gas to form and collect. A trap contains three kinds of rock. The source rock is the fossilized rock that produces the natural gas. The reservoir rock is the porous rock that the natural gas seeps into as it rises. And the cap rock, or seal, is the layer of very dense rock above the reservoir rock that keeps the gas from leaking to the surface.

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