Relics uncovered: Mayan ruins in Guatemala.

Credit: Tom Sever

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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.

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