Could you dangle a tether down from space or dig deep into the Earth and generate electricity from that?

Chris - Well let me do the first bit first and then Jonathan maybe can talk about the temperature bit because that's more his bag. You could dangle a wire from space if you had a satellite on a very long cable. You would have a wire which was passing, if the satellite is orbiting, through a changing magnetic field and if you do that, it will induce a current in the cable, and therefore, you'll separate charges in the cable and you'll have a plus end and a minus end, that's true. But how do you get the energy out? Because you have then got to pass another wire from the plus end to the minus end to get a circuit which means you have another wire passing through exactly the same magnetic field, and it's going to get exactly the same charge separation, so there'll be no net gain. Nice idea, probably very difficult in practice. Jonathan, what do you think about the geothermal part of the question?

Jonathan - Of course, geothermal energy is very, very widely used, particularly in countries which are geothermally active like New Zealand. I think the question though is referring to digging very deep indeed into the Earth's core. This would be incredibly expensive. Deep drilling, going down maybe 1,000 feet is achievable but it's not cheap, and not particularly easy to steer. So, geothermal tends to be restricted to areas where suitable geothermal conditions occur, where heat comes close to surface and can be tapped.