Black and white photos of the sound mirrors by Joe Pettet-Smith on the BBC News website.

More than 100 years ago acoustic mirrors along the coast of England were used to detect the sound of approaching German zeppelins.
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Joe Pettet-Smith set out to photograph all the remaining structures following a conversation with his father, who told him about these large concrete structures dotted along the coastline between Brighton and Dover.

Source: The concrete blocks that once protected Britain, BBC News, 7 January 2019