Scattered across the U.K. today are huge concrete blocks with concave divots cut out of them. Most people who see them don't know their true purpose: A hundred years ago, the British government built these monumental structures as a way to pick up the sounds of approaching airships and know the Germans were coming.
If you think of sustained air raids against London, you probably think of the Nazi blitz of World War II. But that wasn't the first. Instead, the distinction rests with Germany’s zeppelin force of World War I.
Loaded with bombs, these airships crossed the Baltic and North Seas to attack Great Britain, attempting to lower morale and drive Britain out of the war. The could carry nearly three tons of bombs. Powered by four Maybach 3M C-X 6-cylinder inline piston engines, they could reach speeds of up to 57 miles an hour.
Germany began bombing targets in the U.K. in 1915, a time before the invention of radar. British engineers dreamed up a network of concrete blocks, each with a concave surface, deployed in the direction of approaching German aircraft. The concave surface would pick up the drone of airship engines before the human ear could, giving time for air and civil defense forces to prepare for the zeppelin's arrival.
The BBC has a of these unusual devices as they stand today, more than 100 years after construction. They vary widely in size and shape, from 30 to 200 feet high. As the BBC explains, the eventual rise of faster-flying aircraft, coupled with the invention of radar, eventually made the concrete solution obsolete. Now abandoned, they litter the English countryside—and at least one was converted into part of a house after the war.