User blog comment:Darkcloud1111/Good Idea, Bad Idea~Mountain Glenn/@comment-24435688-20141013065426

Have you ever read "City of Ember"?

I see something akin to that situation happening here. The people of Vale expand to mountain Glenn and try to build a new district, but the surface becomes overrun and the flee underground. Some unspecified amount of time passes and the people try buiding their new town underground, but an explosion caused by the contruction crews open up the new cavern full of Grimm.

Then it basically turns into a situation of get out while you can, but if you fall behind you get left behind. One explosion later and the subway tunnels are sealed off and Mountain Glenn is nothing but (almost) empty ruins.