Guide to fracking

The controversial practice of "fracking" - fracturing rock under high fluid pressure - allows oil and gas to be freed from rocks previously considered too tightly-packed to exploit.

Fracking allows drilling firms to access difficult-to-reach resources of oil and gas. The process is carried out vertically or, more commonly, by drilling horizontally to the rock layer.

The energy industry suggests fracking of shale gas could contribute significantly to the UK's future energy needs.