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Shale gas is natural gas trapped within shale rock formations underground. Advances in hydraulic fracturing or "fracking" – the process of forcing fluids at high pressure into wellbores to fracture the shale rock, allowing the gas to escape – encouraged an investment boom in north America.
That led to a big increase in production which brought US gas prices to a 10 year low in April 2012. The price of gas in North America remains far lower than in Europe or Asia.