Some of them say the water was spoiled by drilling deep underground for natural gas.

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But the potential won't be realised until drilling for the underground gas is allowed.

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Scientists have linked the underground injection of oil-drilling wastewater to a magnitude-5.7 earthquake in 2011 that struck the US state of Oklahoma.

BBC: Oklahoma earthquake linked to oil extraction wastewater

Engineered geothermal systems (EGS) are based on a related principle, but they work even in parts of the world that are not volcanically active, by drilling thousands of metres underground to mimic the design of natural steam or hot-water reservoirs.

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More recent techniques apply horizontal drilling to extend operations into adjacent underground deposits.

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Fracking involves drilling horizontally into the rock, forcing sand and water underground to cause tiny fractures that release gas from the shale.

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Fracking is the underground fracturing of very deep underground shale deposits where natural gas is sequestered and then is recovered using normal process procedures of deep and horizontal drilling for recovering natural gas.

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