How does a tidal lagoon power plant work?
Plans to generate electricity from the world's first series of tidal lagoons have been unveiled in the UK.
The six lagoons - four in Wales and one each in Somerset and Cumbria - will capture incoming and outgoing tides behind giant sea walls, and use the weight of the water to power turbines.
A £1bn Swansea scheme, said to be able to produce energy for 155,000 homes, is already in the planning system.
Roger Harrabin has been to Swansea Bay to see how it would work.