Drilling companies suggest trillions
of cubic feet of shale gas may be recoverable from underneath parts of
the UK through a process known as "fracking".
Hydraulic fracturing, or fracking, is a technique designed to recover gas and oil from shale rock. Energy firm Cuadrilla has begun fracking in the UK for the first time since the process was halted in 2011 over earth tremor fears. But how does fracking work and why is it controversial?