[1] The Denge acoustic mirrors, known colloquially as 'listening ears', are located between Greatstone-on-Sea and Lydd Airport, on the banks of a now disused gravel pit.

Several were built along the south and east coasts, but the complex at Denge is the best preserved, and are protected as scheduled monuments.

[3][4] Acoustic mirrors did work and could effectively be used to detect slow moving enemy aircraft before they came into sight.

In 2003, English Heritage secured £500,000 from the Aggregates Levy Sustainability Fund and from the EU's Interreg programme under the Historic Fortifications Network, as administered by Kent County Council.

[5] This money was spent to restore the damage caused by the gravel works, as well as to install a swing bridge which now is the only means of access, reducing the monument's exposure to vandalism.