The Mithraic temples of Roman times were artificial grottoes (spelaea) wholly or partially underground, in imitation of the original selcuded mountain caverns of Asia.
The simplicity and smallness of the Mithraic temples are to be accounted for by structural and financial reasons; an underground temple was difficult to construct on a large scale, and the worshippers of Mithras were usually from the humbler classes.
With this monument as a basis, Franz Cumont has arranged the small Mithraic reliefs into two groups, one illustrating the legend of the origin of the gods, and the other the legend of Mithras.
In the first group are found Infinite Time, or Cronus; Tellus and Atlas supporting the globe, representing the union of Earth and Heaven; Oceanus; the Fates; Infinite Time giving into the hand of his successor Ormazd the thunderbolt, the symbol of authority; Ormazd struggling with a giant of evil - the Mithraic gigantomachy.
Few of the Mithraic reliefs are of even mediocre art.