These recently released radar images were taken by the space shuttle Endeavour in 1994 as part of a geologic study of climate change in northwest China. When Diane Evans, project scientist at nasa’s Jet Propulsion Laboratory, was trying to decide which of the many radar images to process first, her secretary wondered whether the Great Wall of China was visible. After sorting through the images, Evans found that the 600- year-old Ming Dynasty wall was easy to spot: in this image of an area about 430 miles from Beijing, the 30-foot-tall wall is the bright orange line. Evans saw a smaller, less continuous line parallel to it, indicated by the arrow in the black-and-white enlargement. This, it turned out, was the forgotten remains of a much earlier wall, built about 1,500 years ago, during the Sui Dynasty. When Evans’s colleagues in Beijing examined the site, they found that the ancient ruin was barely recognizable as a wall. On the ground, the older wall looks like a jumble of unconnected hillocks, but from space the line of the wall becomes apparent. Says Evans: It was partially buried and so degraded that you’d have to know it was there to recognize it in the field.