Mundus

Mundus patet

As part of the ritual by which Romulus founded Roma antiqua, he set the quadrata atop the Palatine as the center of his City. At its center was dug the mundus that was a concave pit that Cato said represented the counterpart to the concave vault of the Heavens above (Festus 1.c). Popular belief was that this pit was covered for most of the year by the lapis manalis (Festus s. v.). When opened the entrance way to the infernal regions lay opened as well: "Mundus cum patet, Deorum tristium atque inferum ianua patet" (Macrobius, Saturnalia 1.16.18). Offerings were therefore placed in the mundus for Maiores nostrum. Three times a year - 24 August, 5 October, and 5 November, the mundus was uncovered. It was a night when it was believed that the dead emerged to commune with the living.

Sources: Macrobius, Saturnalia 1.16.18; Fowler, W. W. (1899) The Roman Festivals of the Period of the Roman Republic, London.