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