Bridge of Khazad-dûm
The Bridge of Khazad-dûm was a narrow stone bridge crossing a chasm within the eastern gates of the Dwarf-city of Khazad-dûm. It was intended for defensive purposes — enemies who breached the outer defenses would be forced to cross a span of about fifty feet in single file. The depth of the chasm was incredible and the ceiling was high; on the outer side of the bridge was a flight of stairs leading to the exit, through the First Hall and the Great Gates of Moria.
The bridge is most noted for being the location of the encounter between Gandalf and the Balrog. It was broken by Gandalf during the fight but would most likely have been repaired by Dwarves after the War of the Ring, since it was on the main approach to the city from the east.