The 'big ears' of Kent built to keep Blighty safe from European invaders
They were rudely nicknamed ‘Rothwell’s Follies’ by cocky young scientists early in the Second World War, a reference to the talented but older physicist Percy Rothwell, who designed them. Today they’re usually called ‘sound mirrors’ or ‘listening ears’ - and to a certain sort of person they are one of the holy grails of aircraft surveillance.
The Denge Sound Mirrors form a strange cluster of concrete structures on the edge of Romney Marsh, between Lydd Airport and the seaside town of Greatstone in Kent. The three are rare survivors of a chain of acoustic listening devices built between 1914 and 1939 that dotted the coast up to Dover, with a further cluster in north-east England.
Luckily for them they are now marooned on a man-made island in the flooded gravel workings known as the Lade Pits, now part of a huge RSPB nature reserve. The island is inaccessible, reached by an ingenious swivel bridge, securely locked, so the only way to see them is on one of their occasional open days or on a specialist guided tour.
I visited on a brisk, golden afternoon and my RSPB minders were like human sound dishes, alert to the blur of wings over water or the call of a shy bird. Every so often we stopped for them to listen intently and argue amiably over possible avian identities.
Then we rounded a corner and there were the mirrors aglow in the sun: to the south an immense, concave wall, hunched over like a breakwater at the beach but made of reinforced concrete roughly 100 feet long and 26 feet high; next, a vertical structure – imagine a giant shower head on its end - with a perfect half-orange scooped out of the concrete; and to the north a second, polygonal-edged dish almost touching the ground.
What they reminded me of were the astronomical instruments built by Jai Singh II in India, on a modest English scale. What they were there for was to capture the drone of incoming enemy aircraft approaching from the European mainland and focus it onto a microphone or a human listener equipped with then state-of-the-art stethoscopes.
Once the receivers were adjusted for best reception, the results would be compared with those from one of the other ‘ears’ and used to calculate an aircraft’s height, speed and flight path. They worked best at a range of between eight and 24 miles, but by the time the wall was ready in 1930 technology was already on its way towards radar.
Secret Britain | More surprising attractions on your doorstep
We’ve only got them today because English Heritage (now Historic England) stepped in 15 years ago and did the required restoration work. And they are mesmerising: I was expecting ungainly pebbledash, but they had a sculptural beauty made more dramatic by the shadows thrown by their edges. In front of the giant wall was a sloping concrete apron, at the centre of which the microphones once stood, and under the central dish were the remains of a control room where the operator sat listening, intent as a birder.
If you’re really clever, and the other visitors aren’t too noisy, you could do both.
19 beautiful places you won't believe are in Britain
How to visit them
The Denge Sound Mirrors open on Saturday July 7 and Saturday September 1 from 10am to 3pm. Adults ￡5, children ￡2.50, cash on the day. Inaccessible to wheelchairs because of the uneven shingle.
Go to the Lade site and not the RSPB Dungeness entrance, which is three miles away. Stagecoach Buses 11 and 102 stop at Coast Drive, where there’s a car park with toilets (satnav TN29 9PE). From there it’s a 15-minute walk.