Fracking, or hydraulic fracturing, is the process of forcing water, chemicals and sand at high pressure into shale rock deposits underground. The process causes the rock to fracture and allows recovery of shale gas and shale oil, otherwise known as "tight oil", that was trapped in the rock.
Fracking enables wells that would flow only at very low rates to produce oil and gas in commercially viable volumes.
Fracking has been criticised by environmentalists mainly because of concerns that it can contaminate underground water, but also due to other issues.
In April 2011 a gas well in Pennsylvania in the US exploded sending thousands of gallons of fluids laced with chemicals through the drilling site and into a local tributary.
In December 2012 the UK gave the go ahead to shale gas exploration after it was suspended in the wake of two seismic tremors that occurred during fracking operations in Lancashire in 2011.