This is Saemangeum seawall. The Saemangeum Seawall, located on the southwest coast of the Korean peninsula, is the world's longest man-made dyke, measuring 33 kilometres (21 mi). It runs between two headlands, and separates the Yellow Sea and the former Saemangeum estuary.
This park is located beside the Saemangeum seawall. If you're passing through here, you'd better stop by and get some rest. Besides, it would be better if you could find this one.
Have a nice trip.