Natural gas is an odourless, colourless, tasteless, non-toxic clean-burning fossil fuel. It occurs together with a mixture of hydrocarbon gases in petroleum deposits located in deep underground natural rock formations. The gas is mainly a mix of methane plus some ethane, propane, butane, and/or other gases. For years natural gas was a by-product of oil extraction. Technological developments made it possible to extract the gas and store it safely. Before use as a fuel it must be processed to remove impurities including water. It is used as an energy source in heating buildings, generating electricity, providing heat and power to industry and vehicles and is also a feedstock in the manufacturing of products such as fertilizers. The world's largest proven gas reserves are located in Russia, which is the world's largest natural gas extractor. Other large deposits are located in Iran, Qatar, Saudi Arabia and the United Arab Emirates.