When we were preparing for our visit to Ephesus, we were told to look for a symbol of Christianity carved among the stones of the ancient city. According to our guide, the Greek letters for the word fish (ΙΧΘΥΣ) were also an acronym for the Greek translation of Jesus Christ God's Son and Savior (Iēsous Christos, Theou Yios, Sōtēr - ΙΧΘΥΣ). Apparently, the early Christians superimposed the letters on top of each other, creating a wheel with spokes that became recognized as a symbol to indicate where one might find fellow believers without drawing unwelcome attention from those seeking to persecute them.
This photo shows where our guide wrote the letters ΙΧΘΥΣ and then illustrated how they were written together to create the symbol.
In addition to seeing this mark in Ephesus, while we were at the Temple of Apollo in Didyma, we came across the symbol there as well. More information about Didyma and the Temple of Apollo can be found here.