An article about sound mirrors by the security and defence editor of the Financial Times.

Forgotten, dilapidated and, in one case, buried by the local council as an eyesore; these smooth, spherical concrete structures known as acoustic mirrors provided the UK with its first early warning system against German air attack during the first world war.

Source: Joe Pettet-Smith’s photographs of the UK’s early warning air defence system, David Bond, Financial Times, 20 July 2018.

It even quotes this website!