MOUNTAINS DISAPEARING

[cloud3282]

This happening with me too, i disabled the Mods: Misc. Core, Misc. Mai, Misc. Robots and Misc. Incidents and the problem was solved.

Seems like theses MODs has a problem with A15.

[Haplo]

Yes, these mods have a problem when you don't update to alpha 15c as 15a and b have some hash problems with certain kind of mods. Some of my miscellaneous mods fall into this category, which is why I have released them for 15c only.
This error shouldn't appear in 15c anymore.
So I strongly recommend that you update to RimWorld alpha 15c if you haven't done so yet.