Large parts of Rome, specially the hills are made out of Tufa. It's an excellent building material and I reckon that some larger preroman (I can't remember if they were etruscan) burial complexes were build in tufa hills.
I've got a map covering it somewhere here. As for reading up on domes (as I will do even with your ideas Faustus, just out of my own curiosity), I'll see if I can find that after Friday.