Batman?
Oh, wait. Never mind.
I used to own a house that had a cave in the backyard, quite literally. It was on my property, but that little piece of property at the mouth of the cave wasn’t mine because it belonged to the state, and I just owned the surrounding acres. I’m told there’s about a 1,000 foot belly-crawl to get to the first cavern or opening or whatever it is. It took at least 8-9 hours to start exploring the whole thing, apparently.
I’m more than a bit claustrophobic, so I never ventured near it. There were always a lot of spelunkers covered in mud traipsing past my kitchen window, though. They were quite a sight at night, when I’d see this little band of headlights wandering by while I was dining.
You couldn’t charge admission? 
I considered it, because they kept parking on my property. But they seemed to have so much fun crawling into that cave… I sometimes offered to let them use my garden hose to clean off when they came back out.
I thought this had something to do with “Who’s still a virgin?”
That’s awesome MissGypsy, we spelunkers appreciate it! I always felt like I was imposing on the property owners, but they were always cool about it, I guess they got used to it. Some of the caves are way back off the road several miles, but I never noticed trash or any mess around, so I guess the spelunkers are generally a considerate bunch.
Me too!
One of mine, too, although I haven’t been there in 20 years. Hell, I haven’t been caving in 20 years - the friends I used to go caving with are scattered all over the map, now, and it’s not the sort of thing you do on your own.
I’ve been in a few, mostly abandoned mines converted for tourists, but after watching the Cave installment of Planet Earth, I can safely say any future explorations will likely be without my wife. 
I’ve been in Howe’s Caverns and Secret Caverns, both in Howe’s Cave, NY, about 45 minutes from here. It used to be your couldn’t drive anywhere on the east coast without seeing billboards: Only 200 miles to Howe’s Caverns!