Saemangeum is an estuarine tidal flat on the coast of the Yellow Sea in South Korea. It was dammed by the government of South Korea's Saemangeum Seawall Project, completed in April 2006, after a long fight between the government and environmental activists, and is scheduled to be converted into either agricultural or industrial land. Prior to 2006, it had played an important role as a habitat for migratory birds. The completion of this seawall is likely to be a major contributor to the decline of many species. Around 400,000 shorebirds depended on the Saemangeum estuarine as an important feeding ground on the 24,000 km migration between Asia and Alaska and Russia, including the two endangered waders Nordmann's Greenshank and Spoon-billed Sandpiper (each species with less than a thousand surviving birds). A conservation organisation has accused authorities of having failed to monitor the project's impact on local wildlife in a transparent way, and carried out an independent monitoring program in 2006.

The Saemangeum lay at the mouths of the Dongjin and Mangyeong Rivers, on the coast of Jeollabuk-do. It is just south of the estuary of the Geum River. Neighboring districts include Gunsan City, Buan County, and Gimje City. The project of filling in the estuary began in 1991, but was slowed by a series of court actions by environmentalists. The completed seawall is some 33 kilometers long, and replaces a coastline that was once more than 100 kilometers long. After the estuary has been completely filled, an area of about 400 km² (roughly two-thirds the size of Seoul) will have been added to the Korean peninsula, making it one of the biggest land reclamation projects in history. The estuary was originally called "Mangeum". This name was probably formed from combining the first character of "Mangyeong" and that of "Gimje." Saemanguem was completed on 27 April 2010, officially becoming the longest seawall ever built with the length of 33.9 km, breaking the record of Zuiderzee Works from 1932. On 02 August 2010, Saemanguem was certified by Guinness World Records as the longest man-made sea barrier in the world.

The animation below was created using images acquired from the ERS-1/2 and Envisat satellites during their operative phases. This animation shows details of the area in the middle west of South Korea during the construction of the Saemangeum Dam, where the different levels of development starting from May 1992 to September 2010 are recognisable in the images selected.

This multi-layer image is a merge between three different images acquired by the ERS-1/2 and Envisat satellites utilising the SAR/ASAR radar instrument. The images shows the construction of Saemangeum Dam on three different dates. Notable changes in the development during this date range are recognisable in green (ERS-2 26 Mar 2000) and Blue (Envisat 24 Sep 2010), with the artificial Dam and the free economic zone in different phases of construction.

This Landsat 8 image shows the finished artificial dam and the artificial islands still under construction (light brown colour), outside the estuary.

This animation, created with a series of images acquired by the Landsat 5 and 7 satellites, shows the coast area before the dam (starting from 1987) and after the conclusion of the construction (September 2013).

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