The Denge sound mirrors at Lydd-on-Sea are a leftover technology developed just prior to the advent of RADAR. These large concave concrete dishes were designed to collect and amplify the sound of as yet 'out of sight' aircraft approaching from across the English Channel. The faint sound was collected at the centre of the dish where a mounted microphone amplified it for an operator seated in a cubicle. They worked very well but also amplified other sounds making precise identification tricky. The warning time the dishes allowed for was also reduced as planes gradually got faster. Once RADAR was working well the dishes were quickly rendered redundant. The site sits on an island in the middle of a flooded gravel pit which is also a nature reserve owned by the RSPB, and open days are sometimes arranged through them