In The Lord of the Rings: The Fellowship of the Ring, the mine beneath the Misty Mountains is Moria.
Moria is the former underground realm of the Dwarves, also known in Tolkien’s languages as Khazad-dûm. The Fellowship enters it after attempting to cross the mountains through the Redhorn Pass. The abandoned halls contain the remains of a once-great civilization and become the setting for the confrontation on the Bridge of Khazad-dûm.
The film shows that Moria was deserted after the Dwarves awakened a Balrog deep below the earth. The creature is identified as Durin’s Bane, linking the mine’s history to the Dwarf king Durin VI. A common mix-up is calling Moria a fortress or a mountain; it is an enormous subterranean kingdom and mining complex. The Fellowship’s journey through Moria also introduces Gimli to the tragic fate of his cousin Balin’s colony.