Acoustic Mirrors
Very cool concrete "Sound Mirrors" via Boing Boing.
According to the Sound Mirrors website:
Spectacular remnants of a dead-end technology, the three “listening ears” at Denge near Dungeness in Kent are the best known of the various acoustic mirrors built along Britain’s coast. A forerunner of radar, the sound mirrors were intended to provide early warning of enemy aeroplanes (or airships) approaching Britain. They did work, but the development of faster aircraft made them less useful, as an incoming aircraft would be within sight by the time it had been located. Increasing ambient noise made the mirrors harder to use successfully, and then radar rendered acoustic detection redundant.