Why is it so difficult to harness geothermal energy?

Whether you've realized it or not, there is an abundant energy resource bubbling away under people's feet. So-called geothermal energy is stored below the Earth's surface and, if properly harnessed, could potentially solve a lot of the world's energy problems. Despite its huge potential, however, sources of geothermal energy have gone largely untapped. Why? Check out this gallery to find out more.