This is the entrance to a tunnel that was dug under the Roman emperors Titus and Vespasian. Vespasian reigned until 79, then his son Titus took over. The purpose of the complex was to shunt off water that might flood the city that now is Çevlik and formerly was Seleucia ad Piera.
As you can see it is not tunnel all the way. If you go there, be prepared for some climbing over rocks. I seemed to be the only traveller who did not bring a torch. It gets pretty dark halfway, and you might slip and hurt yourself. With light it is still difficult, but less so.