to get at molten materials I believe. Not possible yet but it may be in the future. Also the crust is many layered and moves quite a bit so that a drill bit that deep would probably be severed and the hole constantly refilled.

But you certainly have hit on a great idea and one that I have oaten thought about. The enormous amount of heat contained in under the surface is partially due to the gravitational pull of the huge mass of our planet which gains weight every year from space debris so yes it is renewable.

Looks like tokomak will be the new hot item though in the next 50 years or so.