What a neat idea. I can see a lot of potential in a setting like this, even if it doesn't get added to the GoH (I almost hope it doesn't, so that we can poach it for the LCC!
And you've communicated the idea really well - I like the "double decker" underground look, and the caves and buildings all look solid. I particularly liked the mine carts, and the idea of building normal buildings underground as opposed to using a cave chamber = room paradigm.
If I had one criticism, it's a small one: on some of the minifigures, the blue and green elements aren't quite as integrated as I would prefer - but on some of the others (particularly the leader), it looks great.