Construction has started on Europe's largest man-made nature reserve - transforming Wallasea Island in Essex into marshland.
The island, which is in the Thames Estuary, is being changed from levee-protected farmland into thriving wetland – twice the size of the City of London.
The landmark scheme will cost £50m and is expected to be ready by 2019.
The area will become a place for tens of thousands of migratory birds and marine life as well as helping to combat threats from climate change and coastal flooding.
The past four centuries have seen an alarming loss of coastal habitat and rising sea levels are threatening to see another 1,000 hectares wiped out in the next decade.
The Wallasea Island Wild Coast Project will provide 670 hectares of wildlife habitat, says the Royal Society for the Protection of Birds , which is spearheading the project with Crossrail .
Otters, saltwater fish, including herring and flounder, and saltwater plants such as sea lavender and samphire are expected to flourish there.
Environment Secretary Owen Paterson, who launched the infrastructure scheme, praised it for generating economic growth and helping the environment at the same time.
"This could be the gold standard against which all future projects are measured," he said.
Mike Clarke, chief executive of the RSPB, said: "Wallasea Island will show for the first time on a large scale, how it’s possible to 'future proof' low lying coasts against sea level rise caused by climate change.
"Wallasea Island could now see the return of birds that once bred in England, such as Kentish plovers that were last seen here more than 50 years ago."
Crossrail will send 4.5 million tonnes of clean earth from its new rail tunnels being built under London to restore mudflats, saltmarsh and lagoons last seen 400 years ago.
About eight miles of coastal walks and cycle routes will also be created.
The Government target for the recreation of saltmarshes and mudflats is 3,600 hectares by 2015.
The Essex estuaries are in the top five most important coastal wetlands in the country and are protected by national and European law.
