Urban exploration, urbex or UE, is the exploration of man-made structures, usually abandoned ruins or not usually seen components of the man-made environment. It is also the enjoyment of exploring places that most people wouldn't go or even want to go. Photography and historical interest and documentation are heavily featured. Urban exploration is also commonly referred to as infiltration, although some people consider infiltration to be more closely associated with the exploration of active or inhabited sites. It may also be referred to as urban spelunking, urban rock climbing, urban caving or building hacking.