An international team of archeologists has used radar technology to confirm the Cambodian temple of Angkor Wat was surrounded by the pre-industrial world's most extensive urban sprawl.

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The sun emerges from behind the towers of the famed Angkor Wat temple in Siem Reap province. An international team of archeologists has confirmed the temple was surrounded by extensive urban sprawl. ((Heng Sinith/Associated Press))

In Tuesday's proceedings of the National Academy of Sciences, the researchers report that NASA radar technology has helped reveal an ancient city, hidden beneath tropical vegetation. The city has an area of almost 1,000 square kilometres and is linked by a tightly integrated network of roads and water channels.

The Angkor map is the result of 15 years ofwork by scientists from Australia, France and Cambodia. It uncovers 74 new temples and more than 1,000 new artificial ponds by correlating radar data with on-the-ground sampling.

The use of NASA technology and aerial photography from an ultralight plane helped the team survey areas that were inaccessible due to land mines, a legacy of the 1970s Cambodian war and Khmer Rouge regime. The radar uncovers occupation sites by detecting differences in surface moisture and plant growth and species that are caused by topographical variations due to the presence of architectural remains.

Area supported large population

The study provides the "definitive map" of the Greater Angkor region, the researchers said, andit helps support theories proposed by French archaeologist Bernard-Philippe Groslier in the 1950s, who suggested the area was more than a ceremonial site and that its water network was used to support a large population.

Lead author Damian Evans, of the University of Sydney's Archaeological Computing Laboratory, said the study also supports Groslier's theory that Angkor collapsed because of overexploitation of the land and a breakdown of the water network.

The mapping shows Angkor was "not simply a succession of spatially distinct ceremonial centres, [but] a low-density urban complex like the Classic Maya cities of the Yucatan Peninsula" in Central America, Evans writes.

'It is an engineered landscape that hasn't been matched anywhere else in the pre-industrial world.' —Damian Evans, University of Sydney

"[But there is] no site in the Maya world that approaches Angkor in terms of extent," with the next largest pre-industrial city, Tikal in Guatemala, enclosing just 150 square kilometres.

Evans said the city was serviced by an extensive and sophisticated water system. The reliance on this network could explain the collapse of Angkor as the land was degraded "radically enough to cause them problems," Evans said.

"It is an engineered landscape that hasn't been matched anywhere else in the pre-industrial world."

Angkor was the centre of the vast Khmer empire that controlled much of southeast Asia between the 9th and 15th centuries, before falling to the Thais in 1431.The World Heritage-listed Angkor Archaeological Park is about 300 kilometres northwest of the modern Cambodian capital of Phnom Penh, with the world's largest religious structure, Angkor Wat, at its heart.

Evans said the study has major implications for Angkor's management as a cultural resource, as the remains of the urban complex extend far beyond the designated 400-square-kilometre World Heritage zone that surrounds the central temples.

It also highlights the need to use similar mapping methods on other temple complexes in the tropical world.

"Many of these, like Angkor and the Maya temples, may also lie at the centre of previously undetected low-density urban settlements that are often obscured by vegetation or modern settlements," he said.