Hiya,
That's what I thought when I first heard about high voltage DC transmission line, too -- but they are in fact very efficient, and they are real. There are at least three working HVDC transmission system already, so forget what you think you know on this!
How is a working system that is reliably supplying 15% of the electricity for a major European nation "theoretical"? If they can improve it and enlarge it, it will supply all their power -- if it was "theoretical", why would the German engineers even think of implementing it?
Can you call your local builder to construct a nuclear power plant for you? The temperature at ~8 miles is ~300C, and sure, you could not put an infinite amount of water down there, but the surface area is pretty big, even if you only count the hole itself; and leave out any cracks or fissures.
Science Friday Archives: Geothermal Energy