Energy pioneers have launched the world's first offshore tidal energy turbine off the Devon coast.
The £3m turbine has been built into the seabed about a kilometre and a half (one mile) offshore from Lynmouth.
The single 11 metre-long rotor blade will be capable of producing 300 kilowatts of electricity and will be a test-bed for further tidal turbines.
It is hoped to convert the system to twin rotors by the end of next year.
Project developers Marine Current Turbines Ltd and Cornwall-based Seacore hope to have the turbine hooked up to the national grid by the end of August.
Martin Wright, of Marine Current Turbines Ltd, said: "We estimate that there is at least 10 gigawatts of power available from tidal power in the UK.
"That's the same as about half of the existing nuclear industry."
A marine current turbine looks like a smaller version of a modern windmill and the principle is exactly the same.
The turbine can be raised for maintenance
But whereas a windmill draws energy from the movement of air, the marine turbine uses currents in the water.
Its backers believe the concept can become a rival to wind power because ocean currents are more reliable than wind and also because they are less obtrusive; the structure is built on the seabed and projects just a few metres above the surface.
There is no danger to fish because the blades rotate quite slowly - about 20 revolutions per minute.
Several experimental devices working on similar principles have been built before, but the turbine unveiled in Devon is the first such permanent installation in the world to generate electricity.
If it performs as expected, the consortium behind it hopes to build a whole set of turbines in the area - a tidal farm.
The project is financed by the Department of Trade and Industry and the European Commission's energy programme.