Thus he brought me back at last to the secret ways of Khazad-dûm: too well he knew them all. Ever up now we went, until we came to the Endless Stair.'
'Long has that been lost,' said Gimli. 'Many have said that it was never made save in legend, but others say that it was destroyed.'
'It was made, and it had not been destroyed,' said Gandalf. 'From the lowest dungeon to the highest peak it climbed. ascending in unbroken spiral in many thousand steps, until it issued at last in Durin's Tower carved in the living rock of Zirak-zigil, the pinnacle of the Silvertine.
...
The tower behind was crumbled into dust, the window gone; the ruined stair was choked with burned and broken stone. I was alone, forgotten, without escape upon the hard horn of the world.

(LoTR, III-5)