cave

Bats. Mud, and pools of muddy water. Sweat. Your mate's torchlight flickering before you somewhere (hopefully). Climbing, crawling, squeezing yourself through. Minor attacks of claustrophobia. Major attacks of cold creeping down to your bones. But the sunshine is amazing when you finally get out, and it's the best training your body can get.

(Of course this only applies to non-tourist caves.)