J.R.R. Tolkien calls the Dwarven underground city beneath the Misty Mountains Khazad-dûm.
Khazad-dûm is the Dwarves’ own name for the vast realm later known in Sindarin as Moria. It was founded in the First Age and became one of the greatest centers of Dwarven civilization, wealth, mining, and craft in Middle-earth.
The Dwarves’ excavation eventually uncovered mithril, a precious and exceptionally light metal. Their mining awakened a Balrog beneath the mountains, and the Dwarves abandoned the city after the creature killed Durin VI and later Náin I. By the time of The Lord of the Rings, the abandoned realm is called Moria.
Khazad-dûm is often confused with Erebor, the Lonely Mountain, which is another major Dwarven kingdom. Gundabad and Nogrod are also Dwarven locations, but neither is the famous city beneath the Misty Mountains.