Answer: Acoustic Mirrors
If you wander the coasts of Great Britain, you’ll find strange concrete constructions dotting the coastline, like castoffs of a previous civilization. These curved concrete walls and concrete half-spheres are acoustic mirrors—remnants of Britain’s ambitious World War I-era aircraft detection system.
Prior to the advent of radar-based aircraft detection, the only way to detect an aircraft was to either see it (at which point it was almost on top of you) or hear it (which due to the limitations of human hearing, was almost as bad as waiting to see it). The acoustic mirrors hyper-enhanced human hearing, however, by focusing distant sound waves onto a singular point where a microphone was mounted. Soldiers could listen for incoming aircraft and sound the alarm to scramble out interceptors.
The concrete acoustic mirrors worked well enough for what they were (and the age in which they were first constructed), but the ever-increasing speed of aircraft rendered them obsolete by the mid-1930s. Despite the short effective lifespan of the system, it continued to contribute to Britain’s national defense measures well into the future. The communication network that had been set up for the acoustic mirrors was adapted for communication between early radar towers; that very network gave Britain a distinct edge over Germany during World War II, as German radar stations had no existing network to tap into.