At first glance, geothermal energy seems almost too good to be true. It's clean, inexhaustible, provides predictable 24-hour power and you can get it just about anywhere. There is a snag, however. Outside of geologically blessed places like Iceland, Japan and New Zealand, where volcanically active rocks are close to the surface, the Earth's heat is locked away under several kilometres of rock. Now, though, new developments are making these depths easier and more cost-effective to reach, and the world is beginning to realise the potential of geothermal energy.