World's Heaviest Non-Electronic Eavesdropping Device

The massive concrete acoustic mirrors, or "listening ears," lining the southeast coast of England were built between the world wars to monitor the skies for the telltale sounds of airborne invasion.

Constructed between 1927 and 1930, the sound mirrors were part of Britain's national defense strategy. Their parabolic shape collected and magnified sound waves in the air over the English Channel and directed them at a microphone positioned just in front of the parabola. Anti-aircraft defenses were then deployed. The mirrors effectively gave Britain a 15-minute warning of an impending attack. 

The site features three different reflectors, including a 200-foot-long curved wall, a 30-foot-tall parabolic dish, and a 20-foot-tall shallow dish. All three can be seen in Greatstone, located on the northeast side of the Dungeness Nature Reserve. (more)