How's this for an obvious question: If Google Earth can spot the minivan parked in my neighbor's driveway, why can't a satellite be deployed to spot ancient ruins? That's exactly what NASA's scientists did, when they helped uncover Mayan ruins concealed under the dense canopy of Guatemalan rain-forests.
The space imaging technology was used to identify variations in colors of plants in the region which, it was hypothesized and later confirmed, indicated presence of certain chemicals (like limestone) used in construction of those Mayan structures. Fascinating stuff.
If you live outside the US and don't have access to PBS, you can watch the entire segment on their website. NASA's website has easy-to-understand analysis of the mission along with pictures.