Britannia Mine
Several levels open years ago, but don’t know current situation. Some are hard to find. Biggest accessible mine if you can find ways in, but I understand that the wardens challenge anyone seen going in.
Clogwyn Coch
Nothing underground, stiff walk to get there.
Llanberis
There are 2 main lodes worked, and cut by different levels. First (South Lode) is explorable in its entirety by hairy SRT from the top level. Bottom level is open but grilled, so much if there was a rescue! Go through the gate next to the lay by and its on the right.
North lode has bottom level covered by scree and under water a lot of the time. It may be explorable from engine shaft on hillside. Eldon Pothole Club did a write up in 1983, download from this site. Shame about the difficulties with this mine as there is a lot to see.
The above 3 are listed under “Snowdon” in the late David Bicks book. There are many other copper mines in the north Wales area, many of which are accessible. I have to mention in particular Brynfelin. The adit is regarded as run in, it is not. Clear some leaves and branches and get your wetsuit on. A long crosscut leads to high stopes and other interesting workings.
My avatar is a poor likeness.