Ok Will,
I think that the hill design is a brilliant idea, and about the whole other world thing, make it seem as though you are walking through a small corridor, then emerge into a large pit, which (As the new lore goes) was once used for mining, and some of the passageways are damaged or broken. Another idea is you go through the same small corridor, however you emerge into a cylindrical lab/building that has been placed and suspended in the massive cavern, with (Stained?) glass windows that you can use to see inside. It really depends on what sort of massive thing you want to have stored underground. In my opinion, It is best if you can see it or easily imagine it's size. Like a hill space plane hangar with multiple levels/sections, that can be easily mentally called up and located.
Hope this helps!
- Stardog202