Shale gas extraction: How to get energy from hard rock

The US has almost ended its reliance on imported gas after opening up its reserves of shale gas using the process of "hydraulic fracking".

The gas in shale is trapped in thousands of tiny cracks in the rock, which have to be fractured and propped open chemically - "fracking" - to allow the gas to flow in commercially viable quantities.

But critics say it uses too much water and it is postponing the transition to renewables, and the Environmental Protection Agency is investigating claims the process is contaminating water supplies.

Roger Harrabin travelled to a shale gas plant in Louisiana to find out how technology has allowed more of the gas to be extracted.