Wallasea Island is being transformed from farmland into a 670-hectare (1,500-acre) wetland.
The site is using 4.5 million tonnes of earth excavated from the Crossrail project, for which a 21km (13 mile) tunnel is being bored through London.
The land will be transformed into marshes, lagoons and mudflats to attract birds and other wildlife.
The Royal Society for the Protection of Birds (RSPB) reserve is due to be completed by 2020, and will cost about Â£50m in total.
It is making good use of the excess earth being generated from the separate Â£14.8bn Crossrail project. The twin-bore tunnels being dug out to link east and west London would have seen six million tonnes of earth in need of a new home - but three-quarters of this will head to Wallasea Island via freight trains and ships to create the new reserve.
A new jetty has been constructed to allow the material to be unloaded on the island, which is nine miles (14km) north-east of Southend.
The extra earth will be used to raise the site, which is currently about 2m (7ft) below sea level.
For hundreds of years, ancient sea walls held back the tides to allow this land to be used as farmland. But in 2006, small sections of sea wall were breached to let the waters flood back in, and more will be breached from 2015 onwards.
The RSPB hopes the wetland will attract species such as the spoonbill and Kentish plover, as well as boost numbers of geese, wigeon and curlew.
It also says saltwater fish such as bass, herring and flounder should thrive in the coastal waters.
Paul Forecast, the RSPB's director for Eastern England, said: "The Wallasea Island Wild Coast Project is a landmark engineering and conservation project for the 21st Century.
"It sees partnerships coming together to recreate vital habitat along the Essex coast that was once lost."
He added: "We are really excited to be embarking upon a project of this scale, bringing together a UK major civil engineering project and Europe's largest conservation organisation to secure the future of this magnificent place for people and wildlife for many generations to come."
Construction work has begun on Europe's largest man-made nature reserve, located in Essex.