Caves in the unity engine are impossible.
Unity as it is simply cannot lower terrain horizontally, only vertically.
They would either have to script a tool for it in Unity which would take a long, LONG time, and would be very... hard (now I am not saying Garry can't code that, I am saying it wouldn't be worth the days it would take), build a mountain out of Rock props/models and somehow use that to create them, or make a model of a mountain with Caves carved in, and implement them into the game, which could quite possibly make the game VERY laggy.
Caves are stupid, this game isn't about going into caves and hiding with respawning minerals on the walls or whatever, this is about surviving and rebuilding after a mass event that caused an apocalypse.