Some fifty meters west of the Ka'bah-e Zardusht in Naqš-e Rustam, an ancient, irregular-shaped basin can be seen, cut out into the foot of the mountain. It cannot be dated exactly, but it has been claimed that the workmanship is Achaemenian, which may or may not be true. Creating a basin on this particular site, where the rock layers are almost hostile, cannot have been easy, which suggests that the spot was very important. Unfortunately, we do not know why, and the First Law of Archaeology has been dug up again ("if we don't know what it is, it must have been something religious").
Finally, it must be noted that on at least two places, the rocks have been prepared to add more reliefs, which were not finished. The traces of stone cutting are also visible on several places.
Louis Vanden Berghe, Reliefs rupestres de l' Iran ancien (1983 Brussels), #87.