Spelunking

My favorite natural wonder has long been caves. Ever since I spent a short but key period of my life living just outside of Mammoth Cave National Park, I've been amazed by the world that exists under the ground. Thus far in all my cave adventures, my favorite cave by far has been Gua Tempurung. This cave doesn't necessarily have the biggest underground room, or the prettiest formations, but the tour experience is like no other cave tour I've ever been on, and I've enjoyed it enough to go back 3 times now.
Gua Tempurung offers either wet or dry tours. The dry tours are fairly short and only mildly strenuous (you might only be climbing 1000 stairs). The wet tours are a little more unusual. After reaching the end of the staircases, you head into the lower level of the cave where there are no staircases, no cement pathways, and no lights other than the flashlights you carry. In the lower level, you share the tunnel with the underground river that helped form the cave.
And you get very, very wet. It was only during this most recent tour that I discovered that fish actually live in this underground river--there was one 6 inch fish in particular that wouldn't seem to leave our feet alone, so I was on a mission to stay out of the water as much as possible. Judging from the pictures, I think I succeeded in keeping my head and shoulders dry for the most part.
Some of the passage ways in the cave are no more than 2 feet high. This means that you're doing a belly crawl in the cool water, trying to avoid banging your head on rocks that stick out a little lower than usual from the ceiling, and, depending on the water level, which varies, sometimes significantly, trying to keep your mouth above water as much as possible. 
Basically, it's awesome, and it's as close to real spelunking as I'll probably ever get. Angel and I looked at a "Wild Cave Tour" when we were last at Mammoth Cave National Park, but decided against going because tickets were around $50 and they required us to have actual hiking boots (How crazy is that? Okay, fine, it's sensible).
At Gua Tempurung, the 2.5 hour tour that we took this most recent time costs the equivalent of slightly less than $4 per adult. You don't have to sign any forms or waivers, they recommend that you carry flashlights or else you won't be able to see, but they don't have any types of rules on what kind of flashlights or how many people can share the use of one flashlight. They don't care what you wear--this last time, two of my younger sisters did the whole route in flip-flops (I wouldn't necessarily recommend that.