Known as the 'Listening Ears', these three amazing WWII concrete acoustic mirrors are situated at Denge, behind Greatstone-On-Sea, Kent. Constructed during the 1920s, the mirrors concentrated sound waves of aircraft into a centrally located microphone.
Unfortunately, the operators found it hard to distinguish between ships and planes and as aircraft got faster, the mirrors became redundant. The invention of radar in 1932 also sealed their fate and the area became a quarry.
Now in the hands of the English Heritage, you can only get up-close on a guided tour due to the usual vandalism. A short walk from a small housing estate, these wonderful historic structures make an interesting afternoon visit.