The 12-tonne hub will be filled with resin to stop leaks
An electrical "socket" linking wave energy machines off the Cornwall coast is to be put in place this summer.
The yellow steel structure, about the size of a van, is part of a £42m scheme which is expected to start delivering power to the National Grid in 2011.
It will sit on the seabed in 50m (164ft) of water 10 miles off Hayle and be covered in several metres of rock.
It will feed power from four wave energy machines to an electricity sub-station at Hayle.
The Wave Hub project, developed by the South West Regional Development Agency (RDA), is designed to test various wave energy machines.
Some of them could be used off the north coast of Scotland where the first commercial licences have been announced.
A connection block inside the 12-tonne hub will be filled with resin to ensure it remains watertight and the whole structure is designed to last at least 25 years.
Guy Lavender, Wave Hub's general manager at the RDA, said: "After seven years of planning it's hugely satisfying to see the cable and hub actually taking shape.
"We're on course for deployment this summer and extensive testing will take place before we welcome our first wave energy devices at Wave Hub, which we expect in 2011."
Wave Hub is being funded with £12.5m from the RDA, £20m from the European Regional Development Fund Convergence Programme and £9.5m from the UK government.