DOZENS of protesters, including a member of parliament, were arrested on August 19th during demonstrations around Britain against hydraulic fracturing, or fracking. Green campaigners targeted the offices and drilling sites of Cuadrilla, an energy company, which had to suspend its operations. Fracking has attracted protests everywhere from North and South America to Australia. Several European countries have banned the practice altogether. But how does fracking work, and how dangerous is it really?

By the 1970s America’s energy industry seemed to be in terminal decline. The oil majors had long ago gone abroad in search of richer fields. But a technique invented in the 1940s and adapted decades later by George Mitchell, a Texan oil man, could unlock the oil and gas reserves trapped in shale rock. Mr Mitchell found that by injecting water, sand and chemicals into the ground at high pressure he could fracture the rock and create pathways for the trapped oil and gas to escape. Light regulation and government subsidies have seen fracking take off in America. Shale beds now produce a quarter of the country’s natural gas, up from only 1% in 2000. By last year the price of gas in America had fallen to about a quarter of that in Europe and a sixth of that in Asia, though it has since risen.

Fracking is not without risk. As gas rises to the surface it can escape into drinking water. Recent research published in Proceedings of the National Academy of Sciences, on the impact of drilling in the Marcellus Shale, which stretches from north-eastern Pennsylvania to southeastern New York, found that four-fifths of nearby wells contained methane and that concentrations of gas in the water in nearby homes were far higher than in those further away. Poorly sealed well casings could be to blame. But similar studies of other shale beds have found no methane in nearby wells. Some worry that the subterranean fractures could cause earth tremors: a study linked fracking to small tremors in northern England in 2011 but there is almost no evidence of such tremors in America, where many thousands of wells have been sunk. Greens point out that fracking requires large quantities of water, though other methods of hydrocarbon extraction require similar amounts. Locals worry that more traffic and noise could spoil the countryside. And more research needs to be carried out to determine if drillers are doing enough to prevent methane, a particularly pernicious greenhouse gas, escaping from their wells and pipelines.

A lot is at stake: worldwide production of natural gas is forecast to grow by 50% by 2035, with shale and other “unconventional” sources accounting for two thirds of the growth. Careful regulation can reduce risks by ensuring that well-shafts are leak-proof and that regurgitated gunk is safely collected. (The International Energy Agency reckons that proper regulation would add about 7% to the cost of each shale-gas well.) So far, safety breaches seem to be the exception: a report from MIT found that only a handful of some 20,000 wells drilled in the previous decade had caused contamination, mostly from surface spills of fluids. There are also reasons for greens to support fracking. Natural gas is cleaner than fuels such as coal: America’s carbon emissions fell by 450m tonnes in the five years to 2012, partly because coal was swapped for the gas made available by fracking. In Europe, where expensive gas has led to greater reliance on coal-fired power stations, emissions have not fallen by much. Fracking can be done safely—banning it seems like an overreaction.