A 33-km long sea dyke is being constructed in the Saemangeum area mid-west of Korea to reclaim a surface area of 40,100 ha which includes well-developed tidal flat and two river estuaries. The northern part of the dyke was closed on 10 June 2003, while the southern part was recently closed on 21 April 2006 by plugging two opening gaps of 2.7 km in length. In this study, firstly we review the outlines of key national projects which described marine environments in the Saemangeum area and secondly we present a summary of remarkable changes in marine environments after the closure of the northern dyke, based on marine environmental data collected during 2002-2005. Details in each discipline of the marine environments are presented both in other papers of this special volume and in a series of annual reports of the Ministry of Maritime Affairs and fisheries.