"I've spent 25 years on and off bouncing around in low-level aircraft, searching old maps, looking at aerial photographs, and I found a handful of new archaeological sites.
"I found more sites on the first day of sitting down and doing a systematic survey on Google Earth than in those years of using the other techniques."
As it turned out, many of the Roman villas and other sites Madry spotted online were already known to French authorities.
Though he was initially disappointed not to have made more original finds, Madry soon realized that these confirmations emphasized how quickly and easily promising archaeological sites could be identified from free satellite images.
"The cultural period of these sites [in France] matches exactly with what I thought these things were while I was sitting in my office here in Chapel Hill," he said.
Sparked by the Great War
Aerial archaeology didn't begin with satellites. The concept first took off in Europe during the First World War.
Archaeologists, historians, and other scholars found themselves aloft as part of the war effort. They soon noticed fields dotted with squares, circles, and other unnatural patterns—traces of vanished cultures.
A version of this early work still used today is microtopography: searching for very subtle changes in the Earth's surface relief that may be visible from the air under the right conditions.
Other clues can be found in vegetation.
Stone walls, foundations, and ancient roadways that lie beneath the surface often hinder the plants that grow above them. Their outlines may be seen where plants aren't growing very well, particularly in times of drought.
Wooden structures can have the opposite effect, creating deep, moist soil that boosts the crops growing over their outlines.
Such features are difficult to spot on the ground.
"It's a little bit like holding a newspaper picture right next to your eye," Madry said. "All you'll see are dots."
Now satellites are making aerial prospecting possible on a previously unimaginable scale (explore an interactive feature on satellite use in archaeology).
The technology may even open up parts of the ancient world that are not friendly to aerial observation in modern times.
"In a lot of the world—the Middle East, Greece, Turkey—there are no aerial photos allowed," Madry said. "In a lot of the world you can't just go up in an airplane and fly around or buy some accurate aerial photos."
Seeing What the Eye Cannot
But getting a bird's-eye view is only part of the story. Satellites can also see in ways that that the human eye can't.
Sarah Parcak, an archaeologist at University of Alabama at Birmingham, used satellite imagery to discover more than a hundred previously unknown sites in Egypt's Delta and Nile valleys.
She used images acquired from government agencies and private companies that employ a variety of satellite tools, including multispectral imaging.
The naked human eye can see only a tiny bit of the electromagnetic spectrum, what's known as visible light.
But satellite images can capture visible as well as infrared and microwave parts of the spectrum.
"When I explain this to students, I say that satellites allow us to have a Superman-like vision to strip away parts of the Earth and see things that are typically invisible to us, including subsurface architecture," Parcak said.
"In terms of multispectral imagery, you get some interesting and important work done in archaeology that you can't do [with conventional images]," she said. "So something like Google Earth only scratches the surface of what's possible."
William Saturno of the University of New Hampshire in Durham used multispectral imaging to uncover Maya settlements lost beneath the heavy canopy of the Guatemalan rain forest.
(Related news: "Oldest Known Maya Mural, Tomb Reveal Story of Ancient King" [December 13, 2005].)
Canopy vegetation over ancient sites displayed changes in color and reflectivity that were visible only when captured by satellite instruments in near-infrared spectrums.
"The idea that a tree growing on top of a Neolithic [5,200 to 4,500 years ago] farmer's house is going to look a little different than a tree growing next to that house it's just amazing," said Saturno, who also conducts research at NASA's Marshall Space Flight Center in Huntsville, Alabama.
"The house we're talking about was just a thatched hut on a 50-centimeter-thick [20-inch-thick] masonry platform," he added. "But the trees that grow over it look different when they reflect nonvisible light."
The kind of ultra-high-resolution, multispectral images Saturno and Parcak usually use can be custom ordered from satellite sources such as GeoEye's IKONOS or DigitalGlobe's QuickBird.
For now these images, which can cost thousands of U.S. dollars each, can be prohibitively expensive.
But as free platforms like Google's continue to demonstrate their usefulness and the technology keeps improving, more state-of-the-art imaging could soon be available to all.
"More and more high-resolution imagery is going to be available, and I certainly see the prices coming down in time," Parcak said. "Much of this is going to end up being free."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES