Dwarves built the underground city of Khazad-dûm in J.R.R. Tolkien’s legendarium.
Khazad-dûm, also known as Moria, was the greatest realm of the Dwarves in Middle-earth. It lay beneath the Misty Mountains and became renowned for its mines, halls, wealth, and the precious metal mithril. The city was founded by Durin the Deathless, the eldest of the Seven Fathers of the Dwarves.
In The Lord of the Rings, the Fellowship attempts to cross Moria after finding the Doors of Durin sealed. The abandoned city is dangerous because the Dwarves awakened a Balrog while mining mithril. The creature killed Durin VI and drove the surviving Dwarves away.
Moria is not the city’s original name but its later Sindarin name, meaning ‘Black Chasm’ or ‘Black Pit.’ Khazad-dûm is the Dwarvish name. This distinction can cause confusion, since both names refer to the same famous underground realm.