Yes, that's correct. Except for the first 100 feet or so, the deeper you go into the Earth the warmer it gets. http://en.wikipedia.org/wiki/Geothermal_gradien

Yes, that's correct. Except for the first 100 feet or so, the deeper you go into the Earth the warmer it gets. http://en.wikipedia.org/wiki/Geothermal_gradient http://en.wikipedia.org/wiki/Geothermal_gradient So while the tunnel is at surface level at the entry and exit points, for the majority of the distance it's many thousands of feet deep. It's about 15 degrees F per 1000 feet, so if it's 5000 feet below the average height of the mountain range then you might expect the tunnel to be 75 degrees F warmer than the surface. If the average surface temperature is say 50F then adding 75F gets you to 125F which isn't enough to boil water or generate electricity (efficiently) but it's definitely hot enough to warm a lot of things in a meaningful way.