A 300m section of a sea wall has been breached to begin the creation of the UK's largest man-made marine wetland.
The sea pours through the defence breach at Wallasea Island
Almost 115 hectares has been flooded at Wallasea Island, Essex, to create wetland, mudflats, saline lagoons and seven artificial islands.
The £7.5m government-funded project aims to replace bird habitats lost to development, improve flood defences, and create leisure opportunities.
Excavators were used to breach the sea wall on Tuesday to allow the sea in.
Mark Dixon, who is managing the Wallasea Wetlands Creation project, said the tide spread across land that was once wheat fields and it began the slow process of creating new salt marsh and mudflats.
He said: "It's eventually going to be a new sea defence, so you're going to have brand new mudflats, brand new salt marshes and they'll absorb the tide's energy.
"You've got a big new sea wall at the back, protecting land and property, and then in front of it a series of lagoons and islands and creeks, which birds and people can enjoy."
Biodiversity Minister Barry Gardiner said: "Salt marsh is more rare than rainforest, and is important to people, particularly as a flood and storm defence, and to wildlife.
"Hundreds of thousands of wetland birds rely entirely on the Essex salt marsh for their food each winter.
"Wallasea Wetlands will be a wonderful feeding and roosting habitat for birds like oystercatchers, avocets and little terns, which have been gradually displaced from the area during the last 50 years, as well as creating a haven for other rare wildlife."
It is hoped the wetland will also provide for better fish nurseries.
John Hesp, of Wallasea Farms, said the flooding would help improve the area's flood defences.
He said: "What we're doing here by setting the seawalls back - we call the process managed re-alignment, is that the existing seawalls were in such poor condition, they were simply not sustainable in their present location.
"We've built a new seawall landward and now that we've breached, we've breached at the points where we have the maximum pressure on the estuary.
"So, we've relieved that pressure, enabled the estuary to breathe and we've created more space for water."