I lost a friend in a mine about 10 years ago. He and his buddy were exploring an old mine in Virginia City. He lived there and was very familiar with many of the mines. They got in some "bad" air (low O2), fell asleep, and died. :{ They were within 100' of the entrance. So much for the buddy system. He left behind a wife and twin boys. It happens and you don't know it's happening. Nevada has hundreds of shafts, test holes, and full blown mines. I think it's cool that they're out there, but I think I'd want to go only with air and a VERY experienced guide. My . 02.