Could geothermal energy provide a big missing piece in the puzzle of how to decarbonise the world economy? And do we need the help of oil companies to make use of it?Laurence Knight visits the UK's first ever geothermal power project at United Downs in Cornwall. The project's managing director Dr Ryan Law says it could provide the perfect complement to solar and wind energy, while the resident geologist Hazel Farndale explains how and why they have drilled down more than 5km into Cornwall's granite beds.The last two years have seen a rush of investment and interest in geothermal energy, much of it from the traditional oil and gas fracking industry. Renewable energy journalist David Roberts describes the many innovative new techniques being developed to drill even deeper down into "superhot" rock, including lasers and microwaves.Programme contains a clip from the film There Will Be Blood, produced by Ghoulardi Film Company and directed by Paul Thomas Anderson.(Picture: 3D rendering of the Earth's core and mantel; Credit: Getty Images)