To start out with. Where do your interest lie? I'm guessing you are interested in mines/caves that you could explore to look for crystals and gemstones or meditate in?
Also, what locations are you interested in? The USA, South America, Europe, Asutrailia, Norway etc etc. I'm always interested in information like that as well. There is also a mine somewhere in Norway which is open and free to the public where people freaquently have found crystals. I would have to look it up again if anyone is interested.
You are right about a lot of mines/caves are private or kept secret. I have a few buddies that go caving in the states, and keep some of their locations very secret, even a few which they won't even tell me.
Now there are two important things to remember. Mines are usually man made and not always the safest. I grew up in Pennsylvania where we are known for our coal mines and they often collapse. Caves tend to be safer due to the fact they are naturally and slowly formed over time.
As far as camping in one...I've never caved long enough to have to stay over night in one, and I'd rather not. As far as safety goes, use your best judgement. One thing to keep in mind if you are camping in one is the environmental effect you can have on that cave. Fire's and even food accidently left behind could be very bad for the inhabitants and the ecosystem in that cave.
I'm also fairly new to this community so I'm not sure what all is generally accepted for threads outside of Crystals/Gemstones and their spritual uses. So your question may be better suited in a Bushcraft type forum if you're keen on camping exploring and traveling etc. I'll let a mod decide and figure that out though
Good luck on your journey's!