This is the entrance to a tunnel that is named after the roman emperors Titus and Vespasian (who was his father as you certainly know). They ordered it to siphon water off that would else threaten to flood the nearby city of Seleucia. You can walk (for hundreds of meters) from one side of the hill to the other and then find yourself at a beach. There I did swim, but the sea seemed to contain every plastic bag that ever entered the Mediterranean. A rather horrid experience, since the bags clung to you.