Endless Stair
From Tolkien Gateway
Endless Stair was the winding stairway that led from the deepest halls of Khazad-dûm up to Durin's Tower on the peak of Celebdil. It was extremely long, thus its name.
In the late Third Age it was presumed to have been lost or destroyed and even some Dwarves thought that it was legendary. Gandalf the wizard proved it not so when he pursued Durin's Bane up it to the peak.[1]
