Imagine abseiling in to a cave, taking an underground 'flying fox' even further and then jumping on a black inner tube with a wetsuit on and finish it with a bit of walking through caves older than your grandma. That is the awesomeness of black water rafting. I only know of it in Waitom, New Zealand but it may be in other places or countries.
Nothing freaky about this, just bloody good fun.
http://www.waitomo.com/black-water-rafting.aspx