The last of three programmes on one of the most urgent problems of this half-century. If oil is running out and the safety of nuclear power is in doubt, what else is there? What energy will carry us through the last decades of this century and into the next? Curiously, the answer may be staring us in the face-the sun. It has been heating and lighting this planet since time began. The technology for harnessing it properly is available. Every household could have a virtually free supply of power. In addition to the power over our heads, there is plenty more under our feet. Geothermal energy only a few miles below the earth's crust is already being tapped. But many governments and big corporations are not looking at these things. The research is being done by small groups, often by amateurs. Why? Why do we waste 80 per cent and sometimes 90 per cent of our energy supplies? The solutions are comparatively simple and involve less financial investment, not more. As one scientist says: 'the energy problem will solve itself -either we approach the subject rationally or the lights will go out.' Written and produced by SIMON CAMPBELL-JONES