Basic geography shows that the deeper you dig into the earth the hotter is becomes (and the more pressure too). Some measure it as .7 degree centigrade per every mile journeyed downward. The idea is this, what if we dig deep enough to reach a very high temperature and instead of using water, we could create some synthetic fluid with specialized features such as low boiling point whose vapor has the required pressure characteristics. All we need do is pump such fluid into the ground, get it heated up and it comes back up as highly pressurized gas which can be used to drive turbines for electricity production. At least there is a never ending supply of heat.