The ancient wizardry of Archimedes has been given a new use. The Archimedes screw was invented to raise water up, but of course if you let water flow back down it that would turn the screw in the opposite direction, generating power. Compared to the blades in a modern turbine, an Archimedes screw turns very slowly. This makes it inefficient, but it means that many kinds of fish can swim right through it. So, the greenest sort of hydropower. And it is now functioning at many places in Europe, including Windsor Castle.