If they haven't been destroyed or dismantled, many ancient structures were long ago enveloped by soil, water, sand, volcanic ash, or thick vegetation. Though they might not be obvious to the naked eye, archaeologists are learning how to spot them.

Since the World War I, aerial photography from low-flying aircraft has been widely used. These images can help to pick out relics betrayed by unusual mounds, lines or disjointed landscapes. In other places, buried structures are completely invisible to the naked eye. But they still reveal clues to their whereabouts - just not with visible light.

The human eye can detect wavelengths of light within the range of around 400 to 700 nanometres. But cameras attached to satellites and aircraft are now taking infrared and ultraviolet shots over a much wider range of wavelengths – and revealing some remarkable details about ancient civilisations.

Mayan cities

When NASA's only archaeologist, Tom Sever, looked at an infrared satellite image of a Mayan city in Guatemala, he was intrigued to see the vegetation around the buildings showed up as much brighter than the vegetation in other areas. Following a hunch, Sever, based at the Marshall Space Flight Centre in Hunstville, Alabama, looked for other patches of bright vegetation on the U.S. space agency's maps.

Sure enough, he found additional bright spots at sites not previously considered for archaeological digs.

Sever hypothesised that the limestone that the Maya used for building had leached into the soil, altering vegetation at these sites. Since chlorophyll in plants glows brightly in the infrared range, NASA's satellites were able to pick up the subtle difference in vegetation. With this new method in their toolkit, archaeologists went on to discover several previously unknown Mayan cities.

Spotting entire new cities is one thing, but these images can also provide intricate data about already well-known sites.

Payson Sheets, a professor of archaeology at the University of Colorado, has directed the Arenal Research Project in northwestern Costa Rica since the 1980s. He has used similar infrared images from NASA satellites and aircraft to solve a long-standing mystery.

An answer from the skies

His work revolved around the ancient village of Cañales, settled around 2000 BC in the Arenal area of Costa Rica. The village was abandoned several times after volcanic eruptions. However, after every eruption the village was reoccupied once again. The question of who was reoccupying the village was perplexing: were they the original villagers or new people taking advantage of a prime location?

The answer came from above. Infrared images revealed faint lines stretching between Cañales and a cemetery around 11 km away. When Sheets arrived on the site, he found ancient footpaths – the oldest footpaths known. These had changed the soil texture, and hence vegetation, which showed up as the faint line in the infrared image.

The footpaths were deeply entrenched and found around many of the villages in the area. Sheets argues that they were part of a spiritual ritual for the ancient culture; after excavations, he realised they had been used continually – even after devastating volcanic eruptions.

"The path from the Cañales village to the cemetery was in use over a period of some 800 years, or possibly as long as 1,100 years, interrupted twice by natural disasters from Arenal volcano," he said. "The dominating reason for relocation probably was to re-establish contact with the spirits of their deceased ancestors in their distant cemetery."

Thermal clues

It is not always changes in vegetation that reveals a hidden structure. Every object in the universe continually emits electromagnetic waves, such as light and infrared. The wavelengths emitted depend on the material and the temperature of the object.

Sensitive detectors, on satellites or aircraft, can reveal slight differences in ground cover through tiny variations in temperature. So if a region contains more stone, water or wood than its surrounds, it will stand out in a multi-spectral image.

In August 2007, an international team of researchers used these methods to peel back the ground cover in Cambodia and reveal intricate waterworks around the famous temples of Angkor Wat, the capital of the Khmer empire that flourished between in the 9th and 14th centuries.

As well as iconic temples, Angkor has canals and reservoirs interspersed with small local temples, mounded areas, and ponds. Erosion, breaches in dykes, and evidence of ancient flooding suggest the inhabitants of Angkor were unable to maintain their vast irrigation system.

Archaeologists now suspect the demise of this massive settlement was environmental pressure due to overpopulation and deforestation.

Major expense

Over the last 20 years or so, these multi-spectral images from satellites and aircraft have revealed intricate details and hidden treasures from many more ancient cultures. "I can not imagine doing regional archaeology without good imagery from aircraft and satellites now," said Sheets.

Unfortunately though, this data can be prohibitively expensive. Images from NASA, or from commercial satellites like DigitalGlobe's QuickBird or GeoEye's IKONOS, can cost tens of thousands of dollars each.

In 2005 the Internet search company Google launched its virtual globe – Google Earth – constructed from satellite images and aerial photography. The program quickly caught on with millions of users around the world keen to zoom in on their houses, their dream holiday destinations, and even unintentional pictures of boats and planes.

But then, as the story goes, an Italian computer programmer, Luca Mori, turned its use to archaeology. Using Google Earth, he found signs of a Roman villa buried beneath a riverbed. He contacted experts, who decided to excavate.

American archaeologist Scott Madry, Associate Professor at the University of North Carolina at Chapel Hill, stumbled across a newspaper covering Mori's story. Madry had been professionally surveying archaeological sites for more than 25 years, becoming frustrated with the inefficient, dangerous and somewhat inaccurate method of aerial surveying.

Google it

Within a few hours on Google Earth, Madry was able to locate 101 features in an area covering 1,440 square kilometres in Central France. These features represented Iron Age, Medieval and Gallo-Roman sites.

"I have found a very large number of sites using Google Earth from my office here in the U.S.. I was quite surprised at this, and have now given many different short courses in the U.S. and Europe for archaeologists on how to use Google Earth in their work," Madry said.

"Now, with the commercial ultra-high resolution satellite imagery and especially with Google Earth, archaeologists can conduct regional site surveys from their own offices," Madry said. "This is something we would have only dreamed of before."

All of this does make you wonder: what will happen to the quintessential, khaki-clad archaeologist pictured in countless movies and novels? Will popular culture embrace a relic hunter who looks more like Bill Gates than Harrison Ford or Angelina Jolie? At the rate technology is improving, it won't be long before we find out.

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