The cool thing (pun intended) about this is that the previous clement temperature of the underground was an effectively non-renewable resource. The thermal cap

The cool thing (pun intended) about this is that the previous clement temperature of the underground was an effectively non-renewable resource. The thermal capacity of the earth around the tunnels is so large that it takes a very long time for it to be changed. Once that capacity was used up, it would also take a very long time to get it back. There is a direct analogy with primary geothermal energy, which is effectively non-renewable heat mining. Despite common misconception it is not exploiting the average rate of flow of heat up from the deeper Earth.