As an aside, I have a really hard time understanding why people enjoy spelunking. Extreme cave diving.

I literally get: jumping out of planes, bungee jumping, huge zip lines, extreme heli-skiing, surfing crazy waves, shooting yourself to space, and so forth. That actually makes perfect sense to me.

But spelunking? Putting yourself into a tiny crowded space surrounded by rock? I don't get that.

Now, I've been to a deep coal mine in Pennsylvania. It was fun to explore. But it was a safe thing where we could walk around, etc. So now I know what a coal mine looks like, and get to experience being unusually deep underground. But it's not extreme spelunking, like putting yourself through tiny rock holes.