I'd never seen these things before
Agenda Bender pointed them out via
Apothecary Drawer.
They sit in the south of Britain, these on the coast of Kent. They are massive: the biggest dish there is 30' high, and the big wall is 200' long. They are sound mirrors, made of concrete. They were built in the 1920s. The intention was to listen to what they reflected, using microphones or even stethoscopes, to hear the engines of incoming enemy aircraft.
By the mid-30s someone figured out that RADAR was much better for this purpose, and suddenly the sound mirrors were useless. They fell into disrepair and now there are only a few of them left. This is the biggest set of them, and they are now considered historic and will be preserved.
You can see how large they are by the graffiti.
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