Prior to the invention of radar in the 1930s, a system of concrete acoustic mirrors was set up around the coast of the United Kingdom as an early form of surveillance. These “Listening Ears” were constructed in response to ruinous German zeppelin raids during World War I. Due to neglect and obsolescence, most were demolished. One of England’s six preserved World War I mirrors was just restored in a $100,000 project completed last month.

The Fulwell Acoustic Mirror at Fulwell, Sunderland, was cleaned and outfitted with a new heritage park in a collaboration between Historic England, Sunderland City Council, and the Heritage Lottery Fund’s Limestone Landscapes. Built in 1917 after a 1916 zeppelin bombing in the area killed 22, it was briefly operational and, according to the Sunderland Echo, was used to plan a successful counterattack in 1917. Decades of abandonment followed and the mirror was hidden by overgrown brambles and nettles. In 2011 the decay earned it a place on Historic England’s Heritage at Risk list.

Acoustic mirrors work a bit like a satellite (or your own ears), concentrating sound at a point with the concave shape. When operational, a microphone was situated at this point, and someone would listen in with headphones, potentially giving a 15 minute warning for incoming zeppelins or sometimes water vessels. In 2014, Tim Bruniges’s “Mirrors” installation at Signal Gallery in Brooklyn recreated this technology on a small scale with two concave mirrors on either side of a room, the ambient noise piped back into the space with two giant speakers.

As a mostly forgotten innovation in the development of surveillance, the acoustic mirrors are interesting to revisit, yet the Fulwell Acoustic Mirror is also a case study in preserving concrete heritage. As a porous material, concrete can become difficult to repair and corrodes if neglected (the acoustic mirror’s restoration included diluted sheep droppings as a cleaning solution).

Brutalist architecture in particular is experiencing this wear, such as the Orange County Government Center in Goshen, New York, which was closed in part due to wear on its concrete. Its demolition started earlier this month. Currently Robin Hood Gardens in London with its concrete façade is also in danger of demolition. These are just a couple of examples of historic concrete structures in danger. The acoustic mirror is a much smaller scale project than a whole building, but it does highlight a material that will be central to much of 20th-century architectural preservation.

Fulwell Acoustic Mirror is on Namey Hill near the city of Sunderland, England.