Step into the Void, France
This viewing platform is on the peak of Aiguille du Midi, a 12,650-foot mountain in Chamonix. The rocky summit of this member of the French Alps is intimidating in and of itself, and some people think it resembles a church spire because of the way it towers over the lower slopes. Step into the Void, which first opened in 2013, is a small glass room that hangs out over the edge of the summit.
By "glass room," we mean that the entire box-shaped alcove is made of glass ... including the floor. Visitors who want to "step into the void" have to don slippers so that they don't scratch or scuff the floor. The drop underneath the floor is about 3,400 feet.