The delving of Khazad-dûm is monumental, the densely carved and inhabited parts stretching up six stories above them and seven below, spanning the full breadth of the mountain range and a not insignificant part of its length. Grand staircases transition between the levels, though this level and the one right below are fairly closely intertwined even outside of the largest passageways.
Mines criss-cross deeper into the earth, several times over far below the deepest living space. The entirety, especially the deep mines, has signs of orcish inhabitation, and a few - mostly the first and second levels below them, not the deepest - have proper villages built up in the ruins of the dwarven city. The city proper is hundreds of feet tall; the mines go down for miles -
And then there's cracks, from a few of them. Older, deeper tunnels, the rock that should have filled them simply gone, meander and spiderweb beneath the mountains and into the deepest crust. The most ambitious mine seems to have broken through the top of one unusually high one. She can't feel what made them.
And there's something wandering the lower levels, around where the mines start. Something made of fire, and corruption, and malice, pacing with heavy foot-falls.