Fracking restarted in the UK on Monday – the first such operation since 2011. The oil and gas firm Cuadrilla commenced work at a well in Lancashire after seeing off a last-minute legal challenge on Friday.

The aim is to extract shale gas to contribute to the country’s energy supply but environmental campaigners fiercely oppose it.

What is fracking?

It is short for hydraulic fracturing, a means of extracting gas or oil trapped in rocks deep underground. It is one of the technologies the industry has turned to as new sources of easy-to-extract oil become harder to find.

Fluid composed of water and chemicals, with added sand, is fired at high pressure into the rocks. This creates fissures that allow gas and oil to escape and be brought to the surface.

How big will it be?

That is hard to say. Several companies have bought up vast swathes of shale gas acreage, including Cuadrilla, the petrochemicals firm Ineos and IGas, but none are likely to produce gas at commercial volumes for a couple of years at least.

Estimates of how much shale gas the UK has vary, although the British Geological Survey believes there is more than 3.7tn cubic metres of gas in the Bowland shale – a formation that runs across most of north and central England.

There is far less certainty about how much of it could be extracted economically. Most industry figures are cautious about predicting a British shale boom and comparisons with North Sea oil are widely seen as fanciful.

Why is it controversial?

The Cuadrilla fracking site in Preston New Road, Lancashire.

The Cuadrilla fracking site in Preston New Road, Lancashire. Photograph: Danny Lawson/PA

For a start, any form of gas, while cleaner than energy sources such as coal, is still a fossil fuel and burning it is at odds with efforts to reduce carbon output. There are also specific concerns associated with fracking, such as fears it could contaminate the water table. It has been linked with small earthquakes such as those that took place in Lancashire in 2011, triggering a moratorium on the technique and ushering in tighter regulations.

One form of pollution that is often overlooked is the noise and emissions caused by the juggernauts required to bring water to fracking sites for injection into the ground.

Labour has a policy of banning fracking, the Scottish government has a shale gas moratorium and the Welsh government has promised to block any applications.

The industry argues gas is a key part of the UK’s transition from dirtier fuels such as coal to a low-carbon future, providing a bridge while renewable energy sources are increased.

Will it reduce energy bills?

Despite David Cameron’s claim that fracking would bring down gas prices, few expect the industry to grow to a scale where it has any significant impact on pricing.

The amount of gas likely to be produced from any fracking boom is not going to outweigh the effect of global markets for oil and liquefied natural gas, which the UK imports from Qatar and elsewhere.

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However, it could improve security of supply, insulating against the threat that gas imports could be disrupted or rise in cost because of geopolitical events, such as tension with Europe’s major supplier, Russia.