It's a naturally-occurring mixture of gaseous hydrocarbons which is often found along with petroleum. Natural gas formed millions of years ago, underground, as a result of the same processes that formed petroleum. To obtain natural gas today, people drill into the Earth's crust and tap into pockets of gas trapped there. In the past, humans exploited naturally occurring seepages of this gas, burning it to produce heat and light.

Natural gas is used as a fuel for heating, cooking, production of electricity, and even for powering certain vehicles. Natural gas is usually at least 90% methane, but can also contain propane, ethane, and other lightweight hydrocarbons, in small concentrations. It is lighter than air, and this is a good thing -- it won't collect in low-lying areas if it escapes from the ground or from a pipeline. It has no color or odor, but gas companies add mercaptan, which is really smelly, to it so leaks or open valves (such as on an appliance that someone forgot to turn off) will be noticed. Gas leaks are very dangerous, because if the concentration gets high enough, a spark from flicking a switch -- or even static electricity -- can cause an explosion.

Natural gas started to be used in the 1800s, and at first it was used almost exclusively for lighting of city streets. Eventually pipelines were installed to homes, and electricity became more popular for lighting, so the use of natural gas shifted to heating and cooking.

Five of the 50 states (New Mexico, Texas, Oklahoma, Louisiana, and Alaska) contain more than half of the natural gas reserves in the US.

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