Historic First World War sound mirrors are set to be uncovered by the National Trust on the White Cliffs of Dover after planning permission was granted.

Sound mirrors were one of the first early-warning devices invented in this country to give advance notice of approaching enemy aircraft and came before radar technology.

Work to uncover the sound mirrors, buried under about 600 cubic metres of spoil, has began and is expected to take up to four weeks.

This comes after recent archaeological surveys confirmed that the sound mirror, along with a second dating from 1920-1929, survives buried under Fan Hole at the White Cliffs.

Uncovering the concrete lined structures will be undertaken by three archaeologists and a team of more than 50 volunteers, with all spoil pulled up the steep sides of Fan Hole to be removed.

Some thought the sound mirrors had been demolished in the 1970s until they were rediscovered by the National Trust and local archaeologists on May 3.

Dating from around 1917, the device is the oldest surviving in Kent and one of the earliest known operational sound mirrors to have survived anywhere in the country.

Once uncovered the sound mirrors will be assessed and any necessary conservation work undertaken.

Jon Barker, visitor experience manager at the White Cliffs, said: “With one dating from 1917 and the other being a slightly later prototype, the sound mirrors are a significant national discovery and we hope that visitors will be as amazed as us at their survival.

"To have rediscovered them both and to now be uncovering them is something which our team here will never forget" - Jon Barker, visitor experience manager

“To have rediscovered them both and to now be uncovering them is something which our team here will never forget.”

The excavation work will restore native chalk grassland to the area by removing the rubble and debris that was used to bury the sound mirrors, and has since prevented the natural habitat from thriving.

The restoration project will initially result in some areas of bare chalk, but these will soon return to downland turf with its wide range of wildflowers, butterflies and other species.

Mr Barker added: “During the feasibility study we discovered conflicting information about the survival of the sound mirrors with some records suggesting that they might have been demolished or tipped over.

“It was therefore very exciting when preliminary archaeological surveys showed that the top of both mirrors survived in relatively good condition. We are all very much looking forward to restoring a once prominent feature of this iconic landscape.”