Mines of Moria

Khazad-dûm was the capital and greatest of the mansions of the Longbeard Dwarves in the Misty Mountains. Beneath those Mountains, the Dwarves never ceased their carving and tunnelling, and within Khazad-dûm were great mine-workings. These lay especially away to the north of the city itself, beneath Caradhras, where the famed mithril lodes ran.

There was more beneath the mountains than mithril, as the Dwarves discovered to their cost. Hidden in a deep place was a Balrog that had fled the destruction of Angband, and it was released by the miners of the Dwarves. The Balrog overran the city of the Dwarves, and drove its survivors out into the Wild, leaving the Mines of Moria desolate and dark.