Work to drain millions of gallons of polluted water from mine workings inside a mountain in north Wales has started.
The landscape has been the setting for TV programmes
An acidic underground lake in a former copper mine inside Mynydd Parys on Anglesey is to be drained to avert a potential environmental disaster.
The mine, near Amlwch, has an estimated 50,000 cubic metres of acidic water trapped underground in abandoned workings and shafts.
There are fears over the impact on the environment if the concrete dam, which holds back the water, should give way.
Over the decades, rainwater has seeped through the rocks of the four century-old mine, picking up dangerous metals.
A polluted lake has formed and fears were raised when cracks were discovered in the concrete dam holding back the water.
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 that any flooding would have a major impact on the local area.
"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.
Dave Wagstaff, who has carried out some of the investigation underground, described the scene below the mountain.
"There are pools of acidic water with a pH value of about two, which is like a diluted battery acid.
"The colour of the water is like a very good claret, but if you stayed in it for any length of time, it wouldn't do you much good at all," he said.
Water will be pumped out of the mountain by a team of specialist contractors over a period of four to eight weeks.
The project is being financed by the Welsh Development Agency and Anglesey County Council.
Copper mining had taken place at Parys Mountain for centuries, until it was brought to a halt in 1915.
Since then, its distinctive landscape has proved popular as a location for science fiction films and TV programmes, including Dr Who.