I think one of the most interesting aspects of exploring is that you get to see something as it normally would not be seen, or even imagined. Moss covering old church pews in the light of a sunset gleaming through cracked stained glass windows. A crumbling spiral staircase surrounded by peeling walls. The thing is, you never really know what to expect, because something insane and mind boggling can be right around the corner into the next room.
This is made much more awesome by knowing that at any moment, it could all be gone, and you have gotten to see it.