When researchers at the University of Arkansas started looking through declassified satellite images of the Middle East, they had some idea what they were looking for. They had bought the images, which had been collected by Cold War-era spy satellites and kept classified until 1995, from the U.S. Geological Survey, and began working to match known landmarks and historical sites to the images. But as they were worked they ended up finding much more—10,000 previously unknown archaeological sites.

These sites, National Geographic reports, included roads, canals and even whole cities, likely dating back to the Bronze Age, in Syria and Turkey. The study "tripled the number of known archaeological sites across the Middle East," NatGeo says.

Many of these finds don't show up in modern, sharper satellite images. National Geographic explains:

Current imaging satellites, such as the privately owned DigitalGlobe based in Longmont, Colorado, return better resolution images, but "they can't go back in time," says Casana. The Corona images, he explains, were made before cities such as Mosul in Iraq and Amman in Jordan overran the many archaeological sites near them. Dams have also flooded river valleys, covering many other archaeological sites. As cities grew, the industrial farming and irrigation that supported them grew too, obscuring roads and sites clearly visible in the spy-satellite images.

Corona satellites were America’s first spy satellites capable of taking pictures from space. After the satellites took pictures in orbit, they sent the pictures back in a capsule with a parachute, which were plucked from mid-air by Air Force planes.

The researchers plan to continue looking through the Corona images and are hoping to find sites in other locations, including Africa and China. The researchers compiled their research into an online atlas—where you can see the images yourself.