Archaeologists in Cambodia have found multiple, previously undocumented medieval cities not far from the ancient temple city of Angkor Wat, the Guardian can reveal, in groundbreaking discoveries that promise to upend key assumptions about south-east Asia's history.
The Australian archaeologist Dr Damian Evans, whose findings will be published in the Journal of Archaeological Science on Monday, will announce that cutting-edge airborne laser scanning technology has revealed multiple cities between 900 and 1,400 years old beneath the tropical forest floor, some of which rival the size of Cambodia's capital, Phnom Penh.
Some experts believe that the recently analysed data - captured in 2015 during the most extensive airborne study ever undertaken by an archaeological project, covering 734 sq miles (1,901 sq km) - shows that the colossal, densely populated cities would have constituted the largest empire on earth at the time of its peak in the 12th century.
Evans said: "We have entire cities discovered beneath the forest that no one knew were there - at Preah Khan of Kompong Svay and, it turns out, we uncovered only a part of Mahendraparvata on Phnom Kulen [in the 2012 survey] ... this time we got the whole deal and it's big, the size of Phnom Penh big."
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A research fellow at Siem Reap's École Française d'Extrême-Orient (EFEO) and the architect of the Cambodian Archaeological Lidar Initiative (Cali), Evans will speak at the Royal Geographic Society in London about the findings on Monday.
Evans obtained European Research Council (ERC) funding for the project, based on the success of his first lidar (light detection and ranging) survey in Cambodia in 2012. That uncovered a complex urban landscape connecting medieval temple-cities, such as Beng Mealea and Koh Ker, to Angkor, and confirmed what archaeologists had long suspected, that there was a city beneath Mount Kulen. It was not until the results of the significantly larger 2015 survey were analysed that the size of the city was apparent.
That survey uncovered an array of discoveries, including elaborate water systems that were built hundreds of years before historians believed the technology existed. The findings are expected to challenge theories on how the Khmer empire developed, dominated the region, and declined around the 15th century, and the role of climate change and water management in that process.
"Our coverage of the post-Angkorian capitals also provides some fascinating new insights on the 'collapse' of Angkor," Evans said. "There's an idea that somehow the Thais invaded and everyone fled down south - that didn't happen, there are no cities [revealed by the aerial survey] that they fled to. It calls into question the whole notion of an Angkorian collapse."
The Angkor temple ruins, which sprawl across the Unesco-protected Angkor archaeological park, are the country's top tourist destination, with the main temple-city, Angkor Wat, appearing on the Cambodian national flag. Considered the most extensive urban settlement of pre-industrial times, and boasting a highly sophisticated water management system, Angkor's supposed decline has long occupied archaeologists.
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The new cities were found by firing lasers to the ground from a helicopter to produce extremely detailed imagery of the Earth's surface. Evans said the airborne laser scanners had also identified large numbers of mysterious geometric patterns formed from earthen embankments, which could have been gardens.
Experts in the archaeological world agree these are the most significant archaeological discoveries in recent years.
Michael Coe, emeritus professor of anthropology at Yale University and one of the world's pre-eminent archaeologists, specialises in Angkor and the Khmer civilisation.
"I think that these airborne laser discoveries mark the greatest advance in the past 50 or even 100 years of our knowledge of Angkorian civilisation," he said from Long Island in the US.
There is an undiscovered city beneath Mount Kulen. Photograph: Terence Carter
"I saw Angkor for the first time in 1954, when I wondered at the magnificent temples, but there was nothing to tell us who had lived in the city, where they had lived, and how such an amazing culture was supported. To a visitor, Angkor was nothing but temples and rice paddies."
Charles Higham, research professor at the University of Otago in Dunedin, New Zealand, and the leading archaeologist of mainland south-east Asia, said it was the most exciting paper he could recall reading.
"I have been to all the sites described and at a stroke, they spring into life ... it is as if a bright light has been switched on to illuminate the previous dark veil that covered these great sites," Higham said. "Personally, it is wonderful to be alive as these new discoveries are being made. Emotionally, I am stunned. Intellectually, I am stimulated."
David Chandler, emeritus professor at Monash University in Melbourne, Australia, the foremost expert on Cambodian history and the author of several books and articles on the subject, said the work was thrilling and credited Evans and his colleagues with "rewriting history".
Chandler said he believed it would open up a series of perspectives that would help people know more about Angkorian civilisation, and how it flourished and eventually collapsed.
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"It will take time for their game-changing findings to drift into guide books, tour guides, and published histories," Chandler said. "But their success at putting hundreds of nameless, ordinary, Khmer-speaking people back into Cambodia's past is a giant step for anyone trying to deal with Cambodian history."
David Kyle, an archaeologist and ecological anthropologist has conducted projects at Phnom Kulen, the location of the biggest findings, the massive city of Mahendraparvata, the size of Phnom Penh, beneath the forest floor.
He said the "survey results have revolutionised our understanding and approaches. It's impossible not to be excited. It facilitates a paradigm shift in our comprehension of the complexity, size and the questions we can address."
While the 2012 survey identified a sprawling, highly urbanised landscape at Greater Angkor, including rather "spectacularly" in the "downtown" area of the temple-city of Angkor Wat, the 2015 project has revealed a similar pattern of equally intense urbanism at remote archaeological ruins, including pre- and post-Angkorian sites.
Dr Peter Sharrock, who is on the south-east Asian board at London University's School of Oriental and African Studies and has a decades-long connection to Cambodia, said the findings showed "clear data for the first time of dense populations settled in and around all ancient Khmer temples".
"This urban and rural landscape, linked by road and canal networks, now seems to have constituted the largest empire on earth in the 12th century," Sharrock said.
Evans, whose domain is an air-conditioned room full of computers at the French archaeological centre in Siem Reap, rather than dirt trenches at far-flung digs, is modest about his achievements and quick to credit his colleagues on the Cali project.
A fight scene depicted in detail in the bas-reliefs at the Banteay Chhmar temple complex. Photograph: Terence Carter
He said he believed the discoveries would completely upend many assumptions about the Khmer empire. He also hoped it would bring the study of people back into the picture.
Coe, who has been to many of the places covered by the survey and has seen the imagery, said that while the 2012 survey of Phnom Kulen demonstrated what the technology could do - "it could look through the dense jungle covering these hills and reveal an unexpected city which predated Angkor itself" - the 2015 survey took this into new dimensions.
This view was shared by Dr Mitch Hendrickson, the director of the industries of Angkor project and assistant professor in the department of anthropology at the University of Illinois. He said the initial survey had been "an incredible leap forward" in archaeologists' ability to see everything for the first time and had been "a major game-changer" in understanding how the Angkorian Khmer people built, modified and lived in their cities. But he was "stunned" by the second survey.
"The results for Preah Khan of Kompong Svay are truly remarkable and are arguably the jewel in the crown of this mission. The lidar shows us that there was much, much more," Hendrickson said, referencing a full-blown community layout that was previously unknown. "It's both humbling and exciting. There are so many fantastic new discoveries."
"We knew that Preah Khan of Kompong Svay was significant before the lidar - it's the largest complex ever built during the Angkorian period at 22 sq km, it is connected to Angkor directly by a major road fitted with infrastructure, and likely played a role in facilitating iron supply to the capital.
The team at Siem Reap's École Française d'Extrême-Orient look at a map of the site. Photograph: Terence Carter
"The new results suggest that it may have been more important than many temples built in Angkor and that it had a decent-sized population supporting it."
Dr Martin Polkinghorne, a research fellow in the department of archaeology at Adelaide's Flinders University who is conducting a joint research project on Longvek and Oudong, the post-Angkorian capitals, said his team would use the data during excavations scheduled until 2019 to understand the cities.
"The decline of Angkor is among the most significant events in the history of south-east Asia, but we do not have a precise date for the event," Polkinghorne said. "By using lidar to guide excavations on the capitals of Cambodia that followed we can determine when the kings of Angkor moved south and clarify the end of Angkor.
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"Cambodia after Angkor is customarily understood in terms of loss, retreat and absence; a dark age," he said. "Yet, Cambodia was alive with activity after Angkor. South-east Asia was the hub of international trade between east and west. Using the lidar at Longvek and Oudong in combination with conventional archaeology we will reveal the dark age as equally rich, complex and diverse."
What is a lidar survey?
An airborne laser scanner (ALS) is mounted to a helicopter skid pad. Flying with pre-determined guidelines, including altitude, flight path and airspeed, the ALS pulses the terrain with more than 16 laser beams per square metre during flights. The time the laser pulse takes to return to the sensor determines the elevation of each individual data point.
The data downloaded from the ALS is calibrated and creates a 3D model of the information captured during the flights. In order to negate tree foliage and manmade obstacles from the data, any sudden and radical changes in ground height are mapped out, with technicians who have models of the terrain fine-tuning the thresholds in processing these data points. Once completed, the final 3D model is handed over to the archaeologists for analysis, which can take months to process into maps.