Nowt to do with mining I know, but in my opinion, the best examples of drystone walling are found in some of the Scottish Brochs. Dun Telve and Dun Troddan at Glenelg are amazing. Even with half of each structure robbed out, they still stand about 10 metres high, with the typical outer and inner walls gracefully tapering inwards to support each other. Plus the spaces between the walls provide some excellent above-ground caving!