Hydraulic fracturing, or "fracking", is a technique designed to recover gas and oil which is trapped within shale rock underground. The oil and gas is extracted by drilling down into the earth and using high-pressure jets of water, sand and chemicals into the rock to release the shale gas and oil.
Fracking has transformed the energy industry in the US, where it is expected to account for almost half the nation's natural gas supply by 2035. It is credited with cutting domestic heating bills by up to 40 per cent by reducing the market's reliance on more volatile overseas imports.
Why is it controversial?
There have been health scares in the US which linked fracking to contaminated water supplies, as well as mini-earthquakes and the fear by environmentalists that it distracts governments from investing in renewables.
Where is it done?
Nowhere in the UK, so far, although several sites - especially in northern England - are under consideration for drilling licences.