Answer: Acoustic Mirrors
As you walk along the coasts of Britain you will find strange concrete structures on the coast, such as deposits from an earlier civilization , These curved concrete walls and concrete hemispheres are acoustic mirrors ̵
Before the advent of radar-based aircraft detectors, the only way to detect an airplane was to see it (at the point it was almost over you) or hear it (which was almost as bad because of the limitations of human hearing, than to wait to see it). However, the acoustic mirrors enhance human hearing by focusing distant sound waves to a single point where a microphone has been mounted. Soldiers could listen to incoming aircraft and raise the alarm to eradicate interceptors.
The concrete acoustics mirrors worked well enough for what they were (and the age at which they were first built), but the aircraft's ever-faster speed they were outdated in the mid-1930s. Despite the system's short lifecycle, it continued to contribute to Britain's national defense efforts in the future. The communication network set up for the acoustic mirrors has been adapted for communication between early radar towers. This network gave Great Britain a clear advantage over Germany during the Second World War, as the German radar stations did not have an existing network.