Weekend Posts| Sound Mirror
"Here's one that ought to have archaeologists scratching their heads in the future: huge parabolic surfaces rendered in concrete, dotting the English coast, dating back to 1915.
What are they? "Sound mirrors." In an era before radar, these enormous structures were clever ways to trap soundwaves and hear incoming enemy airplanes before you could see them, and since the craft in question were slow-flying bi-planes, there was still time to do something about it.
British adoption of radar technology in the '30s killed construction of future sound mirrors, but the extant structures were probably too big and troublesome to demolish."
via deputy dog