In 1923 the excavations for the construction of a building of Via Po and Via Livenza led to the discovery of an underground building but inevitably damaged the structures. The Hypogeum had an elongated layout, consisting of a main apsidal hall and some secondary rooms: now has a small trapezoidal portion that it alone can give us an idea of the beauty and uniqueness of this place whose function is still debated by experts.
The north wall is divided into three adjacent arches: the two at the sides gave access to one of the stairs, the other to the now lost small rooms. Over the central arch, richly decorated with frescoes and mosaics and bordered by two marble barriers, is located a deep rectangular tank equipped with a system of adduction and deduction of the water.
The Hypogeum is dated to the second half of the fourth century AD but so far nothing decisive has emerged to explain its actual function.
Fascinating is the hypothesis that it could be a place dedicated to the Thracian Goddess Cotys worshiped by the mystery sect of Baptais, or either an ancient baptistry, because of the base apsidal of the building, the presence of the tank and interpretation of the biblical and symbolic paintings. More thesis argue that this building was a temple linked to the adoration of water, a hidden area destined to magical practices, or because of the coexistence of different influxes, a gathering place for a syncretistic sect. Finally it seems to be a simple grotto constructed in relation to an underground source of water that besides conditioning the architecture and the depth of the building is also the central theme of the pictorial and mosaic decoration. That would explain the coexistence of pagan and Christian elements that are in this building and that culturally fits well in the second half of the fourth century AD, particular historical period characterized by a slow but profound transformation of the Roman world