One of Kent's hidden war treasures has been added to a list of the country's most important monuments.

A pair of acoustic mirrors at Fan Bay, Dover, were used to detect in-coming enemy aircraft and will be recognised by Historic England along with other sites around the county.

They were developed just before the First World War.

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Acoustic detection is where the receiving mirror reflects the sound of distant aircraft engines onto a focal point where it was detected by a listener or by microphones.

The structures at Fan Bay were carved from the cliff face rather than being freestanding objects.

The eastern mirror is one of the earliest surviving examples, dating back to around 1916, and the neighbouring western mirror can be traced to between 1920-1923.

Work to excavate them started in 2014, after they were covered over in the 1970s, when the National Trust purchased the land.

LISTEN: Historic structure protected

The designation has been given because of the rareity, potential to enhance peoples' understanding and the historic importance of the structures.

Historic England's listing team leader for the south, Sarah Gibson, said: "This is a pair of acoustic mirrors which were used as a method of understanding where aircraft were approaching and a very early warning system.

"We schedule nationally important sites" - Sarah Gibson

"Early warning was initially based on visual spotting but acoustic devices were soon developed.

"We schedule nationally important sites.

"Normally it means that any changes to the sit needs to get national monument consent.

"It's a very high level of protection.

"The acoustic mirrors were scheduled in May of this year."

Other sites to be designated historic protection in Kent:

• The Cross, Maidstone, Grade II
• Brasted War Memorial, Sevenoaks, Grade II
• Milton War Memorial, Gravesham, Grade II
• Birchington and Acol War Memorial and memorial enclosure, Birchington, Grade II
• Farnley Little Barn, Ashford, Grade II
• Westgate-On-Sea British Legion War Memorial, Thanet, Grade II
• West Malling War Memorial, West Malling, Grade II
• Dunkirk Parish War Memorial, Swale, Grade II
• Eynsford War Memorial, Sevenoaks, Grade II
• Boughton Aluph and Eastwell War Memorial, Ashford, Grade II
• Fletcher Battery, Eastchurch, Grade II
• War Memorial Cross at the King's School, Canterbury, Grade II
• Westwood Lodge, Thanet, Grade II
• The Cottage, Thanet, Grade II
• Entrance piers, gates and well to Westwood Lodge, Thanet, Grade II
• Nackington War Memorial, Canterbury, Grade II
• Smeeth War Memorial, Ashford, Grade II