Taichi Inoue made this unique touchscreen system. As the pictures at the link illustrate, the screen lies at the bottom of a water tank about eighteen inches deep. But the user needs to insert just two inches of a finger into the water to activate a feature on the screen. Inoue accomplished this by directing a webcam at the surface of the water. The webcam then measures the location and depth of the finger to provide instructions to the computer.
http://taichi.s372.xrea.com/minamo_en.html via Make