Hi mate:
Cligga Head, easy to get into, the cliff you have to traverse and practically climb down in order to get to it, isn't so fun. If you go there be extremely careful. It's an awesome mine though. There are a large number of others near Perranporth, you can get into several.
Some on South Coast accessible, a few around Camborne and Redruth, most of the inland ones are SRT jobs, or simply capped off.
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