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Early in World War 2, before the invention of radar, the British isles had an interesting system devised to detect incoming enemy aircraft. They constructed a number of huge, concrete, acoustic mirrors which focused and amplified sounds coming from a given direction. Some were bowl-shaped and over 30 feet in height, and others were shaped like an amphitheater wall, over 200 feet long.
Inside the structure, a trained listener would use a stethoscope to detect the distance and direction of incoming aircraft as far as 20 miles away. Although they were soon made obsolete by the invention of radar, they were used with success in their time.
A few of these remain in Britain today, and are now being preserved for their historical significance.