There are tunnel systems here, which are warmed from the earths surface, and allow for people to live. It's not comfortable by any means, but it is possible none the less. You are more likely to run across food here than the plains, or mountains outside. Especially in some of the deeper caverns where the planets core provides more warmth. There are underground aquifers, warmed by the planets core, where water can be found, As well as some edible fungi and creatures living in the water here. The chances of finding these tunnels are fairly low if you are new to them. The entrances to here are all found in the mountains.