Work by South Korea to develop a stretch of coastal mudflats has been condemned by conservation groups.
By Alex Kirby
BBC News Online environment correspondent
They say the area is a vital staging post for migrating birds, which will have nowhere else to go.
Work to complete the project presses on
The site is said to qualify for listing as an internationally important wetland.
The South Korean Government says it needs the land for farming and supplying fresh water.
The dispute over the mudflats, at Saemangeum on the Yellow Sea, is described in Dyke Hard, a film in the Earth Report series made by Television Trust for the Environment (TVE) and shown on BBC World.
It says the scheme will be one of north-east Asia's busiest commercial and industrial hubs, occupying an area a third the size of Hong Kong.
Click here to watch BBC World and its report on Saemangeum.
The 33-km embankment central to the scheme will be the world's longest sea dyke.
The conservationists are concerned because of the estimated two million birds which use Saemangeum for resting and feeding during their annual migrations.
Saemangeum is internationally importanrt
Species using the mudflats include the great knot, spotted greenshank (which has an estimated 700 survivors globally), sandpipers and plovers.
The film, which calls Saemangeum one of Asia's most important wetlands, says some birds start their journeys south in Arctic Russia.
After arriving at Saemangeum famished and exhausted they gather their strength and then fly onwards to Thailand, Malaysia and even Australia, which they reach after a non-stop flight lasting three or four days.
Senator Bob Brown, of the Australian Green Party, tells TVE: "I've been to Saemangeum, and I couldn't believe it.
"They're going to flatten more than 100 mountains, fill them in and use the site for profitable industrial or agricultural pursuits, displacing the local people and their fisheries.
"One of the most remarkable feeding grounds is going to be abolished, and with that a huge slice of the migratory bird populations of the Pacific."
South Korea has known famine within living memory, and self-sufficiency in rice remains a priority.
Some visitors are highly endangered
The Ministry of Agriculture in Seoul says Saemangeum is justified because up to 30,000 hectares of farmland are swallowed annually by the spread of the cities.
It says the dyke will both prevent flooding and provide reserves of fresh water.
South Korea supports the Ramsar Convention, the international wetlands agreement. Under its terms, countries are expected to place their significant sites on a list of wetlands judged to be internationally important.
Seoul has not listed Saemangeum, although the Ramsar convention's secretary-general has told Earth Report it richly deserves to be listed.
Friends of the Earth International has written to the South Korean President, Roh Moo Hyun, asking him to save Saemangeum, which it calls "one of the most important tidal flats in the world".
Apart from the birds which use it, FoE says, many Koreans who live from fishing rely on the area for survival.