Fracking, short for hydraulic fracturing, involves forcing up to 10 million gallons of fluid as far as 1,000 metres underground in order to crack rock formations.
The natural gas in the shale below the rock is then channelled back to a well on the surface.
Pollution can occur if seals break in the vertical pipeline underground, which runs through aquifers and other water supplies.
Environmental groups claim fracking can lead to illnesses in humans and animals. It has been suspended or banned in some countries, including France and parts of the US and Britain.
A 'New York Times' investigation found the waste water in some wells contained dangerously high levels of radioactivity.
Among adverse consequences documented in an award-winning 2009 film 'Gasland' were the ill effects suffered by many US residents living near gas wells; flames coming from taps; the destruction of landscape; and instances of water, soil and air pollution.
A US Environmental Protection Agency study also found contaminants in drinking water, including arsenic and copper, adjacent to drilling sites.