We saw this on the map in an area we like to explore, and had to see what there is to see.
Well, the map says cave, but my eyes say mine. Judging by the amount of tailings, not too deep either. Maybe it's dug down into a cave structure. We'll never know though because it's been gated off to allow only bats in or out, not people.
Still a neat place to put a cache. Have fun!