A few zones deeper in the south did not compile right and don't have a rock layer for mining. I have a work around for this that we confirmed works, so if you see you have this issue in your area, PM me (Volcano) and I will create a rock layer for you to use, although it will have bounds / limits until we expand it.
We have 2 players using this method now and so far they said it's been working great.
You will know when you have the issue, you will be able to dig all the way down to water no matter where you are or how hight you are. If you try to mine an uncovered rock face, it will tell you the mountain is too hard, or it will tunnel with a very deep or very high mine entrance which has to be fixed by a gm.
Not a big deal, so far I noticed the risk is higher once you get below 10,800 (Y coordinates), although I have found zones that are just fine in thr south.