I, for one, didn't know before reading this that the temperature gradient is so large, about 25C/km. Being someone who, if I had to guess, has never been more than a hundred metres underground or so, I assumed it was about an order of magnitude lower than that.
The new Case Middle School buildings at Punahou (my alma mater) use something like that, but on a smaller scale. They freeze water at night when electricity is cheaper and then use it for cooling during the day.
Why not, when the mine is done, install a bunch of turbines in the shaft and pour a bunch of water down to 4km, send the steam back up to the top and voila! Endless power.
The fun part is remembering most of geothermal power is derived from nuclear decay. So really, it's nuclear power!
Of course, really, in the end everything is nuclear power. Sunlight (solar panels) is a byproduct of nuclear activity. Fossil fuels are plants that grew by the light of the sun; wind is a result of heating of the atmosphere by the sun, and hydroelectric is a result of heating of the ocean by the sun.
(stretching, I know)... yet the power systems use something more than hydrogen and helium so are made of fusion byproducts from stars. ("We are all starstuff.")
