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If adults [knew|had known] that there [lies|lay] a secret below this peaceful Welsh landscape, they [would have explored|would explore] the place long ago. The point is that there are more than five miles of tunnels [running|run] beneath the picturesque hilly surface. If those tunnels [were not|had not been] so narrow, people would [get|have got] inside the moment they first learned about them. The reality is that only children can squeeze through the holes and [were it not|had it not been] for the kids we [would not know|would not have known] anything about these tunnels. The tunnels are in fact copper mines [firstly|first] dug some 3,800 years ago. Whatever there might be [deeply|deep] inside, these manmade long caves look [curious|curiously]. In any case, nobody looks [indifferent|indifferently] at the first archaeological discoveries [on the site|on site]. Surprisingly, people [did not know|had not known] anything about these copper mines before 1980, when they [had come|came] to public attention. Before the copper mines in Wales and Ireland [went| had gone] into operation, people in France, Holland and possibly Denmark [had not known|did not know] copper. Copper was the second metal that humans [discovered |discovering] after gold and it was [high|highly] desirable in many countries. Already by about the year of 4100 Britain [had learned|learned] how to produce bronze, [mixing|had mixed] copper with tin and [had started|started] the export. The business [took|came] off after the first copper mines [were|had been] found in Britain.