Southern Electric has drilled almost 90ft below Langstone Harbour to lay cables to power 8,100 homes on Hayling Island.
This will end up diverting electricity from the 104ft-high wooden pylon and power lines in the bay.
They have been visible f
or the past 58 years, but the electricity company is to spend £1.6m dismantling them next summer.
Project manager, Chris Smith, said: 'Whilst functioning perfectly well, we were aware that the electricity lines and the pylon in the middle of the water had a limited lifespan due to their age and the nature of their location.
'Underground cables are less vulnerable to inclement weather, so going underground was a logical step forward.
'This work will ensure the integrity of the electricity supply to Hayling Island for many years to come.'
Engineers used remote technology to drill below the seabed and channel cables between the mainland and the island.
The project, which started in April 2009, has been five years in its planning, due to the complex nature of the drilling and the sensitivity of the area around the harbour, which is an Area of Outstanding Natural Beauty, a Site of Special Scientific Interest, and an important area for fish and oysters.
Mr Smith said: 'We've worked with a great many organisations to agree the most appropriate way of carrying out the work, what time of year to do it, how to work with the local sailing community, and how to minimise disturbance to wildlife and the environment in this sensitive area.
'Whilst complex, the first phase of the project has gone to plan, and we hope that when all the work has finally been completed, that people will like the new and improved view of the harbour, with the absence of the poles and lines.'
Hayling historian, Robin Walton, who has lived on the island for 45 years, said: 'It's remarkable to think that when the pylons first went up the island population was probably only a quarter what it is now.'
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