The underground Dwarven city in J.R.R. Tolkien’s The Lord of the Rings is Moria.
Moria, also called Khazad-dûm, was the greatest mansion of the Dwarves in Middle-earth. It lay beneath the Misty Mountains and was originally famous for its mines, halls, and the precious metal mithril. The Dwarf Durin the Deathless is associated with its foundation, and later kings expanded the underground realm.
The Dwarves awakened a Balrog while mining too deeply. The creature killed King Durin VI and drove the Dwarves from their city. By the time the Fellowship enters, Moria is abandoned by its original inhabitants and occupied by Orcs and other dangers. In the Chamber of Mazarbul, the travelers discover the record of Balin’s failed attempt to reclaim the city.
Erebor is the Lonely Mountain, the setting of Smaug’s hoard. Gondolin and Nogrod belong chiefly to earlier First Age stories. Khazad-dûm is the city’s Dwarven name, while Moria is the name most commonly used in The Lord of the Rings.