Listverse has pictures and descriptions of ten strange weapons developed by the Allies during World War II. Among them were sound mirrors, some of which still dot the British coast:
They are the long-forgotten acoustic reflectors, dubbed by locals as the “listening ears.” These lonely structures were built to protect harbors and coastal towns from airborne attacks. Serving as an early warning system, microphones placed at the focal point of the reflector enabled it to detect sounds from flying aircraft over the English Channel, at a range of 30 kilometers. Thanks to acoustic pioneer William Tucker, who helped the radar teams pinpoint enemy aircraft and their movements, the system helped a seemingly inferior, and nearly obsolete, radar system effectively detect German bombers and fighters, and to, ultimately, help win the Battle of Britain.
Link via First Things | Photo by Flickr user Between a Rock used under Creative Commons license