The Hollow Face illusion is created by taking a inverted mold of a sculpture. The faces appear to be pushed out but are actually concave. When you look at it, it would be like looking into a mask except with this illusion it appears to be always facing you. The shadows shift in the mold as you move around it giving it a 3d appearance and its appears to follow you as you move around the room. It can be kinda creepy how it is always staring directly at you no matter where you are.
The first image was taken from an art show in Berlin Germany
The second is a mask of a swedish tennis player Bjorn Borg, this one is located in Uppsala University, Sweden.