Endless Stair

From Tolkien Gateway

The 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 proved it not so when he pursued Durin's Bane up it to the peak, but during the combat the Tower fell and the entry to the stairs was destroyed.[1]