Cagliari, Graffiti from the Roman amphitheater

The graffiti is on the wall of the “Vittorio Emanuele II” tank, located N/W of the Roman amphitheater (1st century BC), inside the Capuchin Garden, attached to the church of Sant'Antonio da Padua on Viale Frà Ignazio.
The tank is part of a water supply system consisting of wells and tanks connected by a series of channels dug into the rock. In the 2nd century AD, the cavity was first used as a quarry for the extraction of blocks and then transformed into a tank, as evidenced by the cocciopesto plaster that covers and waterproofs the walls. Its considerable dimensions (about 130 m deep, 180 m wide, average height of about m 8), allowed a water capacity of about one million liters, and its supply was guaranteed by the rainwater collected from the cave of the nearby Roman amphitheater that, through a duct about 95 m long, reached the tank.
Mauro Dadea hypothesizes that its use as a water reserve has ceased due to serious injuries caused in the walls and that the cavity has been reused as a prison for inmates destined for capital punishment during the shows in the nearby amphitheater. This would be evidenced by the presence of about thirty handles dug into the rock, which would have had the function of supports to chain the prisoners.
Right next to one of the handles you can find some graffiti. Of these, one, engraved on the wall in a very simple way, represents a Roman merchant ship with two masts, in which a series of symbols of Christ are inserted: the cross; the letters of the Greek alphabet P (rho) and X (chi), which form the monogrammatic cross with the initials of the Greek word “Chr (istòs)”; the letters alpha and omega. The symbols of the three theological Virtues are also present (the cross = Faith; the fish = Charity, the anchor = Hope). In addition, twelve vertical bars are graffiti on the bow bridge, interpretable as the twelve Apostles.
Dadea interprets the graffiti as “Navicula Petri”, which in early Christian iconography represents the Church. In the absence of an archaeological excavation, the context cannot be dated with certainty at the moment, even if the author of the discovery hypothesizes that it was made, in the 4th century AD, by a Christian prisoner temporarily detained waiting to be executed, or that it may be pertinent to a hypothetical transformation of the prison hypogeum into a place of Christian worship.
However, such a complex representation of the Church-Ship is rather rare, as it finds a direct comparison with the fresco of the ship in the “Jonah's cubicle” of the Bonaria necropolis.

History of excavations
The graffiti was discovered in 1997 by Mauro Dadea.

Bibliography
A. Floris, Cagliari Underground, Cagliari, 1998, pp. 23-24;
M. Dadea, “The tank of the Capuchin Garden and early Christian graffiti”, in Cagliari. Urban itineraries between archeology and art, Cagliari 1999, pp. 11-13;
M. Dadea, “An early Christian graffiti with the figure of a ship in Cagliari”, in The baptismal building in Italy. Aspects and problems. Proceedings of the 8th National Congress of Christian Archaeology, Bordighera, 2001, I, pp. 155-159;
M. Dadea, The Roman Amphitheater of Cagliari. Sassari, C. Delfino, 2006 (Archaeological Sardinia. Guides and itineraries; 38).