Mack'em 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!