An acidic underground lake in a former copper mine is to be drained to avert a potential environmental disaster.
The lake will be drained and the dam removed
The enormous task of draining millions of gallons of polluted water from the bowels of Parys Mountain near Amlwch on Anglesey, will begin in March.
An estimated 50,000 cubic metres of acidic water is trapped underground in the abandoned workings and shafts.
There have been concerns of the huge environmental implications and a serious flooding risk if the concrete dam, which holds back the water, suddenly gives way.
Over the decades, rainwater has seeped through the rocks of the four century-old mine, picking up dangerous metals.
A polluted lake, which in parts has an acidity level below two pH, has now formed.
The failure could have happened at any time, in ten years time or in a day's time - it was felt it was a risk not worth taking
David Jenkins, Amlwch Heritage Trust
Fears were raised when cracks were discovered in the concrete dam.
Last year, Anglesey County Council was warned the dam was in urgent need of repair, and pledged £20,000 to tackle the problem.
A working group was set up which included the council, the Environment Agency, Amlwch Industrial Heritage Trust, the Anglesey Mining Company and other funding bodies.
David Jenkins from the Amlwch Heritage Trust said a study had shown the effects of the flooding would be "catastrophic".
"The river would flush right through the middle of Amlwch and create major flooding," he said.
"The failure could have happened at any time, in 10 years time or in a day's time - it was felt it was a risk not worth taking," he added.
Pumped out to sea
Water will be pumped out of the mountain by a team of specialist contractors over a period of four to eight weeks.
The water from Parys Mountain is not as polluted as the water already in the river
Head of Technical Services, Mike Barton, explained the water would be pumped to the old settlement pits at Dyffryn Adda before being sent out to Afon Goch river and then to sea.
Ian Cuthbertson from Anglesey Mining plc supports the project and says it is the most comprehensive of the plans considered.
"Draining through Afon Goch north will restore the mine to the condition it was in 20 years ago," he said.
"The water from Parys Mountain is not as polluted as the water already in the river - it is the lesser of two evils," he said.
Water from the Afon Goch Dulas river is the second highest contributor of toxic metals to the Irish Sea.
The project will be financed by the Welsh Development Agency and Anglesey County Council.