After a quarter of a century of scouring the Burgundy region of France for potential dig sites that would illuminate the area's rich cultural history from the Iron Age to the present, archaeologist Scott Madry was accustomed to a sluggish pace of discovery. His best available option was taking to the skies in a small plane, to reach a vantage point where faint geometric patterns became visible: straight lines, right angles and circles that bore evidence of ancient roads, walls and buildings. But such flights were expensive and only possible on the clearest of days. Even then, the plane's constant motion made it difficult to stay oriented.

This all changed in late 2005 when Madry discovered Google Earth's trove of high-resolution satellite imagery. From the comfort of his office at the University of North Carolina in the United States, he freely ranged over an area nearby the one he'd studied, as if pilot- ing a virtual helicopter. Geometric patterns indicating the presence of ruins were everywhere. "I saw clear evidence of Roman villas, pre-Roman Celtic farmsteads, medieval earth-walled fortified farms," Madry says. "Within a few days, I found more sites than I had found in years of doing aerial and field surveys. It just knocked my socks off."

Upside-Down
To sample the power of earth- imaging satellites, anybody can simply boot up Google Earth and zoom in. Depending on the part of the globe you've chosen, the view may be close enough up to reveal individual cars and trees. The source of many of these astounding aerial shots: satellites orbiting the earth at more than 7 kilometres per second with their high-resolution cameras trained on the ground hundreds of kilometres below. (Cameras mount- ed on planes also supply much of the close-up imagery on Google Earth.)

The satellite images on Google Earth are patched together from a variety of different sources. Broad landscape views are provided by satellites such as NASA's Landsat series, which produce full-colour imagery at a resolution of 30 metres, meaning that each pixel in an image represents a square on earth measuring 30 metres by 30 metres. Landsat satellites have been snapping images of earth since 1972, and Landsat 5, one of the two currently in orbit, just celebrated its 25th year of labour - not bad for a mission initially set to last only three years.

Google Earth's higher-resolution satellite imagery comes from a series of advanced commercial imaging satellites that have been launched in recent years. The current champion is GeoEye's GeoEye-1, launched in September 2008 with a Google decal affixed to its carrier rocket. This can produce full-colour images with a resolution of just 50 centimetres (at this resolution, it's possible to tell the difference between a car and a truck). GeoEye-1 has just begun feeding imagery to Google Earth, but DigitalGlobe's QuickBird, with a full-colour resolution of 61 centimetres, has been beaming imagery down since its launch in 2001.

Spotted Plants
While Madry uses satellite imagery to reveal what's right under his nose (and under the ground), conservation biologists are using it to locate undiscovered habitats far off the beaten path. The resource has been invaluable to the efforts of Britain's famous Royal Botanic Gardens, Kew, to survey and conserve mountain- side forests in southeastern Africa that tend to be rich in indigenous flora and fauna but increasingly under threat by development.