I was told by someone who visited Mountain mine a couple of years back that, as part of the restoration of the spectacular engine house, the main adit level had been cleared. It was a simple and easy "walk-in" job to extensive workings with big stopes open to surface, much iron-mongery associated with the engine house above, large stemples, colourful coatings on the walls and stopes full of water fouled by dumping of rubbish from above.
I do not know the current situation, but if the level is still open then it sounds like a great place to visit. As I understand it there are other mines in the vicinity too.