The Domitian Fountain, later turned into the Pollio Nymphaeum. Wikipaedia on that: N.. was a monument consecrated to the nymphs, especially those of springs. These monuments were originally natural grottoes, which tradition assigned as habitations to the local nymphs. They were sometimes so arranged as to furnish a supply of water, as at Pamphylian Side. A nymphaeum dedicated to a local water nymph, Coventina, was built along Hadrian's Wall, in the northernmost reach of the Roman Empire. Subsequently, artificial grottoes took the place of natural ones. The Tomb/Fountain of Pollio was erected in 97 AD in honor of C. Sextilius Pollio, who constructed the Marnas aqueduct, by Offilius Proculus. It has a concave façade. The Polypheme Group was found here, now in the museum.