Poll: What for you personally is the scariest thing imaginable?

I was in Eastern Russia exploring some caves. I had my hand on the side of the wall with some friends, and we walked deep into it, at least 300 yards. We were in total darkness, with only weak flashlights. We tried to cross, but couldn't find the other side of the cave! The feeling of being in a n open space, in total darkness was the scariest thing that I've ever experienced.