Fracking, or hydraulic fracturing, is the injecting of high-pressure water containing chemicals and sand into deep underground rock formations to create cracks that allow the previously nearly inaccessible natural gas and petroleum trapped there to flow freely and be retrieved.
Although some forms of fracking have been in use since the 1940s, improvements in the technology have made it practical for energy companies to retrieve oil and gas that was formerly inaccessible. In some countries, most notably the U.S. and Canada, fracking has grown rapidly in the past several decades and oil and gas obtained using it have become a substantial share of their energy output. This has reduced their reliance on imported fossil fuels including coal, which is a much greater source of greenhouse gas emissions per unit of energy produced than oil or gas.
However, fracking remains highly controversial and has been banned in some countries because of its possible adverse environmental and health effects. In addition to the damage to the environment resulting from the burning of the fuels obtained through fracking, these effects include the risk of pollution of ground water by the fracking fluid, leakage of toxic gasses and greenhouse gasses directly into the atmosphere, and the possible facilitating of or causing earthquakes. Studies have shown that elderly and low income people living near or downwind from fracking and other oil and gas extraction sites are more likely to die prematurely, presumably because of the air and water pollution and also possibly because of the noise and light pollution.