Beginning in the 1940s, the Soviets mapped the world at seven scales, ranging from a series of maps that plotted the surface of the globe in 1,100 segments to a set of city maps so detailed you can see transit stops and the outlines of famous buildings like the Pentagon (see above). It’s impossible to say how many people took part in this massive cartographic enterprise, but there were likely thousands, including surveyors, cartographers, and possibly spies.

Most of these maps were classified, their use carefully restricted to military officers. Behind the Iron Curtain, ordinary people did not have access to accurate maps. Maps for public consumption were intentionally distorted by the government and lacked any details that might benefit an enemy should they fall into the wrong hands.

Davies and Kent argue that the maps were a pre-digital Wikipedia, a repository of everything the Soviets knew about a given place. Maps made by U.S. and British military and intelligence agencies during the Cold War tended to focus on specific areas of strategic interest. Soviet maps contain plenty of strategic information too—like the width and condition of roads—but they also contain details that are unusual for military maps, such as the types of houses and businesses in a given area and whether the streets were lined with greenery.

Exhaustive notes on transportation networks, power grids, and factories hint at the Soviets’ obsession with infrastructure. Davies and Kent see the maps not so much as a guide to invasion, but as a helpful resource in the course of taking over the world. “There’s an assumption that communism will prevail, and naturally the U.S.S.R. will be in charge,” Davies says.

Very little is known about how the Soviet military made these maps, but it appears they used whatever information they could get their hands on. Some of it was relatively easy to come by. In the U.S., for example, they would have had access to publicly-available topographic maps made by the U.S. Geological Survey (legend has it the Soviet embassy in Washington, D.C. routinely sent someone over to check for new maps). To obtain more obscure information, they would have had to get creative.