To map a landscape's history, foresters long depended on maps and traditional tree inventories that could be riddled with inaccuracies. But now they have a bird's-eye view that is the product of a 20th-century U.S. spy program: the Corona project, which launched classified satellites in the 1960s and '70s to peer down at the secrets of the Soviet military. In the process, these orbiting observers gathered approximately 850,000 images that were kept classified until the mid-1990s.

Modern ecologists chronicling precious or lost habitats have given second life to the Corona images. Paired with modern computing, the space-based snapshots have helped archaeologists identify ancient sites, demonstrated how craters left by U.S. bombs during the Vietnam War became fish ponds and recounted World War II's reshaping of Eastern Europe's tree cover.

Even though they're static, the panoramic photos contain discernible imprints — penguin colonies in Antarctica, termite mounds in Africa and cattle grazing trails in Central Asia — that reveal the dynamic lives of earthly inhabitants below. "It's Google Earth in black and white," said Catalina Munteanu, a biogeographer at Humboldt University of Berlin who has used Corona images to show that marmots returned to the same burrows throughout decades of destructive agricultural practices in Kazakhstan.

Modern systems like the Terra, Aqua, Copernicus and Landsat satellites provide environmental scientists with regularly updated images of the planet's surface. But those satellites have been around only for a few decades — four, at most — and many offer less-detailed resolution than the photographs recorded by Corona.

More important, with the spy satellites, scientists can extend a landscape's timeline even earlier into the 20th century. This, paradoxically, helps us predict what comes next.

"When you double or triple the age of that record," said Chengquan Huang, a geographer at the University of Maryland, "you can substantially improve your modeling ability into the future."

Once wrangled, Corona's spy photos can uncover a landscape's history beyond the contemporary era of widespread satellite imaging. Often, Corona's '60s-era snapshots captured habitats before humans drastically inundated, paved over, plowed up or developed wild spaces into new cities, hydroelectric dams, farmland or industrial zones. The images even challenged our assumptions about untouched ecosystems — revealing, more than once, that presumed old-growth forests are actually younger than 70 years old.

"In a lot of cases, they lead us to landscapes that are gone, that don't exist anymore," said Jason Ur, a Harvard University archaeologist. "Corona is like a time machine for us."