These maps were captured in the waning days of World War II as the U.S. Army took control of Japan. American soldiers confiscated thousands of secret Japanese military maps and the plates used to print them, then shipped them to the United States for safekeeping.
The maps covered much of Asia, and they went far beyond the local topography. They included detailed notes on climate, transportation systems, and the local people. It’s the kind of information that could be used to plan an invasion or an occupation, and some of it was gathered by spies operating behind enemy lines. To the Japanese, these maps are known as gaiho¯zu—maps of outer lands.
To the Americans, they were a valuable source of intelligence, not just on a recently defeated foe, but also on a newly emerging one—the Soviet Union. The Army Map Service considered it unwise to hold such an important strategic resource at a single location that could be wiped out in a nuclear strike, so it distributed the maps to dozens of libraries and institutions scattered across the country.
And there they remained, virtually forgotten, for decades.
Read more at National Geographic