Non-Mineable areas in Mountains.

[erdrik]

The idea is to force the player to adapt their building plans when met with an area they can't mine and build into.

My thought is, similar to uncovering a cave or open area, you would uncover a seemingly bottomless pit.
Or maybe just a cave lake. Something that prevents the player from building there, so they have to change/adapt their plans around it. It shouldn't act as support for the mountain roof either, so they have to be careful about how they dig around it.

[Wex]

Cave lakes would be easy to add in the game as it is, for sinkoles it would require some modding, I guess.

[Neotic]

+1 this would be nice as it would force players to work around it but it might be annoying. What if the farther you go into a mountain the marble/sandstone/etc will have higher hit points