Mount Qaf (Kaf Dağı) stands at the edge of the world in Turkic and Iranian tale, encircling it. There is no crossing it; beyond is outside the known world.
In the tales, what lies behind Mount Qaf is the place of trials: the Simurgh lives there, the padishah's youngest son goes there, and whoever returns is not the same person. For something to be "behind Mount Qaf" is still the idiom for out of reach.
In Balbal it is both a floor (Mount Qaf, fourth in the Sky) and a skeleton — a board with a mountain silhouette, also used on the Nowruz and Ghoul Desert floors.