Sound mirrors or 'listening ears' are large concrete constructions built during the second world war as an early warning system for enemy aircraft. Sound would be collected in the concave dishes and listened to by trained personnel. The system was never actually used, having been superceded by radar technology.
Danish artist Lise Autogena is building two of these sound mirrors, one each side of the channel between England and France. By standing on a platform infront of the sound mirror you will be able to hear and communicate with your counterpart on the other side of the channel.
What an amazing idea. More info on the original constructions can be found here.