First off: My suggestion isn’t concerning Z-Levels, so don’t bite my head off lol.
Uh…hills and mountains. Kind of like how we have caves made out of rock, BUT HUGE! HUGE!
(Yes, double emphasizing)
I want to be able to dig a dwelling out of one, so they should be pretty damned big. Like a 3x3 on the map. They would be just huge solid rock spawns. Hell, they could even randomly have caverns/caves in them.