The Dwarven underground city called Moria in J.R.R. Tolkien’s Middle-earth is Khazad-dûm.
Khazad-dûm was the greatest mansion of the Dwarves and was built beneath the Misty Mountains. Its name is in the Dwarves’ own language, known as Khuzdul. After the Dwarves awakened a Balrog while mining for mithril, they abandoned the city, and it became known to others as Moria, a name associated with darkness and dread.
The Fellowship of the Ring travels through the ruined city after attempting to cross the mountains. Their journey includes the Chamber of Mazarbul, the Bridge of Khazad-dûm, and the confrontation between Gandalf and the Balrog.
A common mix-up is treating Moria and Khazad-dûm as different places. They are the same city: Khazad-dûm is its Dwarvish name, while Moria is the better-known later name used by other peoples. Erebor, by contrast, is the Lonely Mountain, another important Dwarven kingdom.