You won't need to dig too deep to discover the area's mining history. Check out King Arthurs Labyrinth for some insight into the precious resources once excavated here.
This inactive mine site in Machynlleth comes recommended by travelers, so make a spot for it on your travel plans. Don't think that's all there is to do here, though. There are plenty more places waiting for you to appreciate after King Arthur's Labyrinth. Cader Idris Mountain, Centre for Alternative Technology, and Owain Glyndŵr Centre are not far away.