Endless Stair
On the peak of Zirakzigil, also called Celebdil, the Mountains of Moria were overlooked by Durin's Tower, carved out of the rock itself. From there, a set of spiral steps descended into the mountain, and continued downwards without pause through all of Moria, into its deepest depths. This was the Endless Stair.
Gimli suggested that the Endless Stair had been lost to the Dwarves, but the Balrog of Moria knew where it lay, and used it to escape from the pits beneath Khazad-dûm. He was pursued by Gandalf up to the Tower on the mountain's top, and there they fought the Battle of the Peak in which the Balrog was destroyed.