One of the most intriguing enigmas is about the mausoleums of the 12 emperors of the 257-year-long dynasty.

History suggests these emperors were buried after their death in areas around the city of Xi'an in today's Shaanxi Province in Northwest China, but none of their mausoleums have been found, despite thousands of years of efforts by burglars and archaeologists.

Archaeologists say that the major reason for this was that cultural practice at that time forbade imperial mausoleums from being marked, combined with a lack of reliable historical records.

Chinese archaeologists have made several large-scale field investigations over the past decades, only to discover tomb relics of a few noblemen.

So far, the sites of the imperial tombs remain a mystery.

New light

A new explorative project is now going on around the suburban areas of Xi'an using the latest technology that archaeologists hope will shed new light on this enigma.

Remote sensing, a technology that has been introduced into archaeological exploration in China only in the last decade, has been used to identify the sites of these mausoleums, according to archaeologists who attended the first national Remote Sensing & Archaeology Symposium in Beijing last week.

Archaeologists have hired planes to tour several regions southwest of Xi'an that are hypothesized to encompass the burial sites of the Western Zhou emperors over the past few months, taking photos of the landscape from mid-air.

By analyzing these photos, they hope to find clues to the Western Zhou mausoleums that might otherwise be easily ignored through ground observation or field investigations.

"The conventional methods used by archaeologists for hundreds of years have proved insufficient in detecting these mausoleums," said Zhu Fenghan, archaeologist and director of the Chinese Museum of History. "Remote sensing has given us a new perspective to recover the traces of relics we might have overlooked."

The conventional methods Zhu mentioned are analysis of historical records and field excavation, both of which failed to pinpoint the actual sites of any of these mausoleums.

According to Zhu, initial clues to their whereabouts come from ancient historical writings suggesting the first emperor of Zhou Dynasty, Wu, was buried at "Bi," a place presumed to be near today's Xi'an, but which has never been identified. This theory has been advocated, and challenged, by a variety of subsequent historical writings that put forward conflicting hypotheses.

Several candidate areas were put forward around Xi'an, none of which has been confirmed.

Archaeological excavations were systematically carried out over last century, particularly in later times, but these were to no avail.

Zhu is optimistic about the new attempt, which, however, may take years to achieve any concrete results after sorting through the photos to correctly read, examine, and confirm each "unusual spot" with field excavations.

But such effort is worthwhile given the historical implications of these mausoleums, which archaeologists believe may hide the answer to many queries about Chinese ancient culture.

The Western Zhou Dynasty is generally regarded as a time when Chinese traditional culture was taking its shape.

The Rules of Zhou, for example, were created then to dictate both social practice and individual behaviours and their influence on Chinese minds continued over thousands of years until today.

Art also reached it zenith, when bronze ware making reached a level unsurpassed both in technical and artistic terms.

But interestingly, little bronzeware made in Zhou Dynasty has circulated on the antique market, where old bronzeware is commonly available.

According to historical records, bronzeware was an essential part of the artefacts accompanying the deceased Zhou emperors in their mausoleums.

Discovery of the bronzeware, which often bears inscriptions depicting the life of the emperor, would probably rewrite the history we used to take for granted.

"It is probably a good thing that we now have so little bronzeware from the Zhou Dynasty," Zhu said. "It means that burglars probably have not broken into these mausoleums over the past thousands of years."

Crop marks

Zhu led the first team of aerial photographic archaeology in China to do archaeologist excavation.

Aerial photography is one of the most often used means of remote sensing technologies in archaeology, he said.

This is also the first time this technology was used in the detection of cultural relics in the main area of Chinese civilization, he said.

Aerial photography and observation has resulted in hundreds of photos that revealed a landscape featuring a great deal of human activity, he said.

Although human activities over thousands of years have significantly altered the landscape, the former ancient constructions may still have indirectly left their mark on the environment, allowing for observation from a distance, he said.

For instance, an underground relic, such as a tomb, may be discovered by comparing the ground vegetation right above it with that in the surrounding area, as their colours and shapes may differ greatly due to the difference in the soil caused by human activity, Zhu said.

The soil over a tomb is generally looser and better than the surrounding natural soil, thus allowing for better growth of vegetation.

The difference in vegetation often constitutes so called "crop marks" that may not be easy to notice on the ground. Rather, they are usually distinguishable by observation from mid-air, he explained.

Such crop marks have led to sensational discoveries in Europe and Central America, according to Guo Huadong, researcher with the State Lab of Remote Sensing and Archaeology.

Crop marks are not the only clues to such discoveries, he added. Soil, sun shades, frost vestiges may all help distinguish the large area of cultural relics from the surroundings, he noted.

The mausoleums of Zhou emperors were typically constructed near their palaces, according to historical records, which suggests their relics would probably be large enough to leave various distinguishable marks.

"We do not expect to accurately pinpoint the sites by any single mark or by any single means," Zhu said.

For example, other remote sensing technologies, such as coloured infrared light detection, may prove particularly useful to detecting metallic artefacts such as bronzeware.

With the combination of field excavation, we will probably not have to wait for another century to see these mausoleums come to light, Zhu said.

(China Daily December 26, 2002)