During the Cold War, the Soviet military undertook a secret mapping program that’s only recently come to light in the West. Military cartographers created hundreds of thousands of maps and filled them with detailed notes on the terrain and infrastructure of every place on Earth. It was one of the greatest mapping endeavors the world has ever seen.

Soviet maps of Afghanistan indicate the times of year certain mountain passes are free of snow and passable for travel. Maps of China include notes on local vegetation and whether water from wells in a particular area is safe to drink. The Soviets also mapped American cities in remarkable detail, including some military buildings that don’t appear on American-made maps of the same era. These maps include notes on the construction materials and load-bearing capacity of bridges—things that would be near-impossible to know without people on the ground.

Much of what’s known about this secret Soviet military project is outlined in a new book, The Red Atlas, by John Davies, a British map enthusiast who has spent more than a decade studying these maps, and Alexander Kent, a geographer at Canterbury Christ Church University.

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.

In this map of San Diego, the added detail may have come from satellite imagery, which the Soviets had access to after the launch of their first spy satellite in 1962. In other cases, detail may have come directly from sources on the ground. According to one account, the Russians augmented their maps of Sweden with details obtained by diplomats working at the Soviet embassy, who had a tendency to picnic near sites of strategic interest and strike up friendly conversations with local construction workers. One such conversation, on a beach near Stockholm in 1982, supposedly yielded information about Swedish defensive minefields—and led to the Soviet spy being deported after a Swedish counterintelligence agent lurking nearby overheard the conversation.

Exactly how the Soviet maps came to be available in the West is a touchy subject. They were never meant to leave the motherland, and they have never been formally declassified. In 2012, a retired Russian colonel was convicted of espionage, stripped of his rank, and sentenced to 12 years in prison for smuggling maps out of the country. In researching the book, Kent and Davies had hoped to speak with some of former military cartographers who worked on the maps, but they never found anyone willing to talk.

As the Soviet Union broke up in the late 1980s, the maps began appearing in the catalogs of international map dealers. Telecommunications and oil companies were eager customers, buying up Soviet maps of central Asia, Africa, and other parts of the developing world for which no good alternatives existed. Aid groups and scientists working in remote regions often used them too.

For anyone who lived through the Cold War there may be something chilling about seeing a familiar landscape mapped through the eyes of the enemy, with familiar landmarks labeled in unfamiliar Cyrillic script. Even so, the Soviet maps are strangely attractive and very well made, even by modern standards. “I continue to be in awe of the people who did this,” Davies says.