We had some interesting times on the one expedition I did. It was fascinating and I would recommend trying it at least once... doesn't have to be dangerous. Even going to Carlsbad Caverns, which is a National Park and while not the real spelunking experience, pretty cool. I went to Wolf River Cave in Tennessee. Most of it was just like mountain hiking, but with a ceiling. Questionable parts included crawling in light mud on our hands and knees for 600 feet through an area where the ceiling was about 3 feet high. Also one part, you go through a 'door' and have to drop down ~5 feet onto some rocks... people told me "be sure to go left when you land!!" and wtf was to the right? This giant dark pit of rocks at least 20 feet deep. Okay... then at the very bottom, there was this area with a bunch of trickling water and awesome stalagmites where you could sit on rocks by this weird little stream and ponds. We split up and sat in different rooms... the guy from Kentucky I sat with, who I'd never met before, told me "sometimes when I'm down here... i listen to the water... and it sounds like people talking..." Uh, okay.

But anyway it was an amazing experience and profoundly strange... the 'rooms' and 'hallways' are oddly reminiscent of human construction. And if you get stuck or hurt, if you've done things properly and signed in and people know you're there, experienced cavers will come and rescue you.