An ancient Roman ruin.   It appears that the column on the left was added in more modern times in an effort to preserve these ruins. This was one of the most impressive bathing facilities that the world has ever seen.  It was called the Temple of Venus, but it was actually part of the nearby thermal bath complex that was built by some of the most noteworthy  figures of Roman history. It is now separated from the other ruins, that are part of the Archaeological Park of Baia, by the modern Via Lucullo. 

    To learn more about the Ancient Baths of Baiae check out the article in Palimpsest: An Anthology of Italian History & Culture. or Take the Tour of the Ancient Bath Complex!