The long island of Kekova south of Üçağız is famous for its spectacular underwater ruins. These are partly submerged 6m below the sea off its northern shore, and are referred to as the Batık Şehir (Sunken City). The ruins are the result of a series of severe earthquakes in the 2nd century AD; most of what you can still see is a residential part of ancient Simena. Foundations of buildings, staircases, moorings and smashed amphorae are visible.
It is forbidden to anchor or swim around or near the Sunken City, but kayaking is allowed. Some locals use the name Kekova to refer to the whole area.