I have a mountain range that separates two places that certain people will want to go between fairly regularly. Right now they have to travel around or over a mountain range to do that.
I've been thinking about ideas of how to make traversal easier and quicker and one possibility that occurred to me was a cave(rn) system that could be entered on one side and eventually exited through the opposite side, similar to what Tolkien did with the Mines of Moria.
The difference here, however, is that Moria was excavated while I'm supposing this to be a natural formation. I don't know enough about geology to say whether or not this is absurd.