Diga di Luzzone, Switzerland
Though it is not a purpose-built wall, Diga di Luzzone stands as the Everest of wall climbing. The route from the base to the top of this dam in the Alps stretches for more than 500 vertical feet. Exposed to the elements and climbing higher above the ground than on any other human-made route, people who tackle Luzzone have to negotiate five pitches, each one more challenging than the last. Though climbers won't have much time to enjoy the view, it is undeniably scenic here, with the dam surrounded by Switzerland's trademark mountain landscapes. Unlike many of the other walls on this list, Luzzone does not have nearby options for novice climbers. For those seeking the ultimate non-natural climbing challenge, however, there is no other wall that comes close to this one.
Check out the video below of climbers taking on the wall. Warning: Might induce sweaty palms.