Blaenau Ffestiniog has some really cool slate mines;
Cwmorthin
Wrysgan
Rhosydd
There are more of course but they are great for a wander.
Cwmorthin is locked and you get a key from "Go Below" (
www.go-below.co.uk/) You leave a £20 deposit which they will give back to you when you return the key.
Wrysgan and Rhosydd are both open and you can just wander in (Go Below have the lease for Wrysgan but no gates).
There are also a few smaller copper mines on the banks of Snowdon itself - Hadod-y-Porth is nice for a wander but it is wet (goes over wellies) and it is a little hike to get to it.
There is a nice copper mine (Drws-y-Coed) on the other side of Snowdon over by the Nantlle Ridge - that is very pretty but quite short.
There is also the Britannia Copper mine on Snowdon itself (the miners track takes you right to it). This is stoped and all the floors are missing - you go in the bottom entrance and wander around. It's very pretty but it is gated. Last time I looked the gate was open with no lock. I don't know if it still is. Water also goes over wellies at the entrance.
If you travel to Betws y Coed, there are several very pretty lead mines;
Parc Mine
Pandora Mine
Aberllyn Mine
All are accessible and all are good. Parc is the biggest but not the prettiest - access is easiest though (and it's dry apart from about 50 feet by the entrance)
Aberllyn is also dry but has collapsed on the 4th floor so you cannot descend below that - the entrance is also a real bugger to find. It is pretty thouggh.
Pandora is also pretty. Easy to get in but watch out for a false floor at the first main junction. Expect water over the knees for the first 200 feet too.
We are on facebook here;
www.facebook.com/groups/30320727365/
Have a peep at us there too.
If you want specific details of any of the above let me know and I can give you grid refs/directions. I am useless with Google and can't work out how to stick pins in images. If you Google the names of the mines I have given you - I think you will get the info to find the entrances. Come back to me with any questions too.
Also, depending on the date/day you want to actually go underground, I may be able to arrange someone to go with you ... possibly .... maybe ...
Ian