I was informed these are the poor remnants of the gorgeous Nymphaion (2d century A.D.)
From the Wikipedia: A nymphaeum or nymphaion (Greek: νυμφαίον), in ancient Greece and Rome, 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.