Engineers at the University of Electro-Communication in Tokyo designed the AquaTop, a touch-sensitive and responsive interface that works in a pool of water:
AquaTop, which has its own dedicated operating system, recognizes specific gestures. Spread your fingers apart for a full-screen version of "Waterworld." Scoop the water to drag an image, release the liquid to drop it. Need to delete something? Just "sink" it by bringing one hand down.
Modified audio speakers make sound waves to create feedback that users can see on the surface and feel through the water. It's simply amazing stuff. Skip to 4:10 in the video to get to the eye-popping effects.