As I was reading ”Alice in Sunderland” (a fantastic book), I came across a reference to a World War 1 early warning system - an acoustic mirror.

During WW1, Sunderland was bombed at least twice by Zeppelin. As an important shipbuilding town it was a great target, and the acoustic mirror system was deployed over the North East to protect the industrial areas from attack.

This particular example is a grade 2 listed structure, and a scheduled monument. The City of Sunderland website has a good overview.

It’s not every day you learn something new about your hometown!