It depends on the type of mine you are looking at storing in and the location.
I'm a "retired" mining professional and I have worked in the USA and visited mines in several foreign country's. Some mines are extremely wet and humid, others are dry.
I would think that any mine that is being used for underground storage has decent conditions and possibly they even condition the air/ventilation. Things that I would be looking at is access, decent road conditions to get in and out. I would think that there would be power supplied to maintain batteries. You might consider putting in some heat lights in the engine compartments and house to keep moisture away.
Check to make sure the environment isn't acidic, what was mined. If the air smells like sulfur it is corrosive and will eat metal quickly.
Also, look for what they do for roof support. Most mines use some sort of roof bolting to maintain safe roofs but over time they can deteriorate and become ineffective. Look for any fresh signs of where rocks have fallen from the roof. This can happen by the variation in temps, expand/contract. If the temperatures are fairly constant this will not be as big of a problem.
I don't know the specific rules/regulations that these storage facilities have to abide by. In active mines MSHA rules/regs are very stringent. No gas or propane motors and even with diesel engines there are minimum ventilation requirements.
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