In between the two world wars, a precursor to radar was built in Britain. This took the form of large concrete bowls to act as acoustic mirrors to listen for the sound of oncoming planes from the continent. Microphones would have been placed in the sweet spot of the reflector, and by moving the mic the direction of the plane could be told with a high degree of accuracy.
Rendered obsolete with the invention of radar, these dishes sit on an island in a nature reserve, inaccessible without a boat. I, however, happen to have a boat...
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