ix Kal. Sept. (Aug. 24). Mundus Patet.
This does not appear in the calendars. We learn from Festus[[909]] that on this day, on Oct. 5, and Nov. 8, the ‘mundus’ was open. This mundus was a round pit on the Palatine, the centre of Roma quadrata[[910]]—the concave hollow being perhaps supposed to correspond to the concave sky above[[911]]. It was closed, so it was popularly believed, by a ‘lapis manalis’ (Festus s. v.). When this was removed, on the three days there was supposed to be free egress for the denizens of the underworld[[912]].
I am much inclined to see in this last idea a later Graeco-Etruscan accretion upon a very simple original fact. O. Müller long ago suggested this—pointing out that in Plutarch’s description of the foundation of Roma quadrata the casting into the trench of first-fruits of all necessaries of life gives us a clue to the original meaning of the mundus. If we suppose that it was the penus of the new city—a sacred place, of course—used for storing grain, we can see why it should be open on Aug. 24[[913]]. Nor is it difficult to understand why, when the original use and meaning had vanished, the Graeco-Etruscan doctrine of the underworld should be engrafted on this simple Roman stem. Dis and Proserpina claim the mundus: it is ‘ianua Orci,’ ‘faux Plutonis’[[914]]—ideas familiar to Romans who had come under the spell of Etruscan religious beliefs.