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Energy firm Cuadrilla began fracking in Lancashire in the north of England after a legal challenge and multiple protests failed to stop work.

The process of fracking involves pumping chemicals at high pressure, deep underground, to fracture rock and release natural gas. It was temporarily banned in the UK in 2011, after tests were linked to minor earthquakes.

The drilling technique is used in several other parts of the world but remains highly controversial. Professor Richard Davies, a petroleum geologist at Newcastle University, answered our questions.

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