Whether in the form of crumbly rock, odorless gas or slick black liquid, ancient organic compounds have powered the flourishing of modern life. But now they threaten to make the planet unliveable for many plants, animals and humans.
The collective term refers to coal, oil and natural gas — a group of energy sources that contain carbon and hydrogen.
They started life hundreds of millions of years ago as living plants and animals. When they died, they sank to the bottom of oceans and swamps where they decomposed under layers of sediment.
Over time, this matter was buried deeper and deeper into the earth, subjecting it to increasing pressure and heat which changed its chemical composition. Plants and trees transformed into coal, whereas microscopic plankton and algae became oil and — if the heat and pressure increased further — gas.
Today fossil fuels can be found in the earth's crust all around the world although some countries — such as the US, Saudi Arabia and Russia — have bigger reserves than others.
Coal, found in underground coal beds, is extracted by mining, whereas oil and gas, found in reservoirs beneath the land and ocean, are accessed by drilling.
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