I have heard this asked many times. The question is mainly "What equipment do I need, and what equipment should I buy?". The suggestions may be good, but the main ingredient is often overlooked because because old mine exploration is treated as a hobby.
The main rerquirements are the person(s) participating and their common sence!
I have worked as an exploration geologist for plus 50 years where I have worked first at an operating mine and later investigating ancient mines in dating back to old Inca mines and on occasion "lost" mines. Through experience I have had a few misshaps and made mistakes, and learnt from all of them.
I do not care what equipment you want to buy, but I will mention some pointers that you may need to know.
1. Any mine, active, ancient or this century should be explored with caution. Rock fails, the ledges at stopes may fail, the back (roof) may collapse, etc. Step carefully and test your weight, especially in wet mines.
2. Observe the condition of the back at all times in case of scaling or possible falls - the major cause of accidents in mines.
3. In old abandoned mines be aware of the air. In old days miners with even with carbide lamps carried canaries in cages to give the first signs of bad air. These days, and especially with electric head lamps, carry a lighter or matches to test the air. You will not notice when you first get lethargic from lack of oxygen. If your candle is burning low, get the hell out of there !!!
4. Many people do not realise this, but a unless you have worked often in a mine that has used explosives, you may get "powder headaches" after a while underground or slightly later. Unpleasant and a temporay inconvenience.
5. If you think that the air is a bit "off" but still good, rerstrict your time underground.
6. Be sufficiently fit and alert, so that if a misshap happens you can get out of it.
The above cautions are more important than what equipment you buy.
If it cannot be grown, it has to be mined.