Angus M.
Google
This place is outside of the main tourist draw of the city centre, but well worth the detour.
You'll be entering the mine in a mine cart, which is a unique experience by itself. Once inside, it is really a wondrous thing to wander the empty tunnels, seeing all the equipment rusting away. While not a natural cave, the rust, graphite veins, glitter of fool's gold, and limestone stalactites is a magical sight.
The only downside is that our guide did not speak English. While there is an information paper, judging from our Czech speaking co-visitors, this really is no replacement for the guide itself.
Even so, the mine tour is very recommended. Make sure to book ahead (by phone call or email, other staff speaks English) during the busy months. Also, make sure to bring something to keep you warm: it is 10 degrees Celsius year round down there.