When a new town was founded, a round hole would be dug and a stone called a lapis manalis would be placed in the foundations, representing a gate to the underworld.
Due to similar names, the lapis manalis is often confused with the lapis manilis in commentaries even in antiquity:
The "flowing stone" … must not be confused with the stone of the same name which, according to Festus, was the gateway to the underworld.
Bailey (1907) states:
There is, for instance, what anthropology describes as 'sympathetic magic'—the attempt to influence the powers of nature by an imitation of the process which it is desired that they should perform. Of this we have a characteristic example in the ceremony of the aquaelicium, designed to produce rain after a long drought. In classical times the ceremony consisted in a procession headed by the pontifices, which bore the sacred rain-stone from its resting-place by the Porta Capena to the Capitol, where offerings were made to the sky-deity, Iuppiter, but from the analogy of other primitive cults and the sacred title of the stone (lapis manalis), it is practically certain that the original ritual was the purely imitative process of pouring water over the stone.
Read more about this topic: Manes