Yes, there are spoilers here. Go away if you're one of
those people.
For anyone who might not know (and I'm sure almost everyone does), the mountain has at least three stripes of features the deeper you go in: an underground river, a bottomless(?) chasm, and a boiling lake of lava.
To be blunt...what is past the lava? I will cross it and find out at some point, but progress is rather slow and it'd be nice to know what's out there. The intro movie is perhaps more informative than it ought to be?
I hear that you must cross it with a steel bridge, and to make steel you need to make iron and pig iron first. What are the ingredients in making pig iron and steel?
I have visions of a netherworld hell-throne surrounded by grim totems and bones. Or perhaps it's just a series of small pockets populated by goblins. Either way, every time I imagine crossing the lava I hear Saruman talking about how the dwarves "dug too greedily and too deep."
I also thought of a neat feature, in case it is not already implemented: coming across an already-mined tunnel that is partially collapsed and hearing someone calling for help. You unearth a legendary miner who is starving and dehydrated and nearly insane, babbling about ghastly creatures and warning you to delve no further.
Of course, this is all assuming there really is something fearsome in there and not just more veins of copper.