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.

In 1991, the South Korean government announced that a dyke would be constructed to link three headlands just south of the South Korean industrial port city of Gunsan, 270 kilometres (168 mi) southwest of Seoul, to create 400 square kilometres (150 sq mi) of farmland and a freshwater reservoir. 

Since then, the government has spent nearly 2 trillion won (1.82 billion USD) on construction of the dyke, with another 220 billion won (199.89 million USD) budgeted on strengthening the dyke and a further 1.31 trillion won (1.19 billion USD) to transform the tidal flats into arable land and the reservoir. 

The average width of the sea wall/earth dam is 290 metres (950 ft) (it is 535 metres (1,755 ft) at its widest) and the average height is 36 metres (118 ft) (54 metres (177 ft) at its highest).

According to en.wikipedia