Nidavellir

One of the Nine Worlds, Nidavellir is an underground realm located beneath Midgard, yet above the Underworld. It is often referred to as the Realm of the Dvergar, though numerous other peoples also lay claim to it - including Svartalfar, goblins, and many more. Perhaps even larger than the world above it, its many tunnels and vast rooms are home to entire civilizations and ecosystems never seen by dwellers of the surface.