The Cave of Krilos (The Test of Time Supplement)
From D&D Wiki
Dark and ancient these tunnels are, its air has not been stirred for eons and its inhabitants have never seen the sun.
Ancestral home of the dwarfs, this plane consists of tunnels and enormous caves (so large that they hold entire cities). This plane knows no surface or sky, just rock and tunnels and a lot of it. These tunnels run through the entire plane and are usually well kept, but obviously there are exceptions like tunnels that collapsed because of earthquakes or battles, ancient tunnels very deep in the earth where no mortal has been for centuries and trap set up to slow or prohibit travel through certain areas.
The beings that live there are dedicated and strong because it takes a lot of effort to survive here for a long time, and the most thriving species are the dwarfs because of there endless patients and magical prowess. but they are not the only ones, there are many threats like the "fallen dwarfs" or "Dueregar" that live in the extreme depths and Xorns that can glide through the rock without effort.
Meta-game description: mix between "the portal of death: abarrach" (world of stone) and "Lord of the Rings: Moria"