[wcoenen]

> Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious.

When the London metro was built (over a century ago), the clay was at 14 degrees C. But over time, it heated up from all the power dissipated by the metro system. Now it is around 20 to 25 degrees C and doesn't really act as a heat sink anymore. The same happened in the NY subway.

So soil can act as a great heat sink for a while, but not forever. I suspect the same would happen if you'd put a modern city underground and relied on the temperature of the soil instead of pumping heat out somehow.