In the year 802 C.E., the founder of the medieval Khmer empire, Jayavarman II, anointed himself "king of the world." In laying claim to such a grandiose title, he was a little ahead of his time: It would be another few centuries before the Khmers built Earth's largest religious monument, Angkor Wat, the crowning glory of a kingdom that stood in what is today northwestern Cambodia. But Jayavarman II had good reason to believe that his nascent kingdom, in the sacred Kulen hills northeast of Angkor, was a record-holder. Airborne laser scanning technology, or LiDAR, has revealed the imprint of a vast urban landscape hidden in the Kulen's jungle and in the lowlands surrounding Angkor Wat; by the 13th century, the low-density cityscape covered an area of about 1,000 square kilometers.

The findings show that the cityscape at the heart of the Khmer Empire of the 9th to 15th centuries C.E. was much more sprawling and complex than archaeologists realized and lend weight to the hypothesis that, strained by climate change, the complexity of the kingdom's vast waterworks was its ultimate undoing. The LiDAR revelations are "astonishing," says Roland Fletcher, an archaeologist at the University of Sydney in Australia and a member of the international team whose findings are in press at the Proceedings of the National Academy of Sciences.

At its height, the medieval Khmer empire encompassed much of modern-day Cambodia, central Thailand, and southern Vietnam. Archaeologists have long inferred that Angkor was the most extensive city of its kind in the pre-industrial world. Its singular achievement was a complicated network of waterways and reservoirs that were apparently vital to producing enough rice to sustain a population that in the center's heyday numbered in the hundreds of thousands.

Through painstaking ground and aerial surveys and excavations, Fletcher and colleagues in recent years have uncovered evidence that Angkor's waterworks began to break down around the time that the kingdom faded from the historical record. "Things are going wrong by the 1300s," Fletcher says. Signs of severe distress include massive sand deposits in canals and the ruins of a spillway that the Khmers may have ripped apart themselves. In 2009, tree ring data indicated a potential culprit: a decades-long period of megamonsoons and droughtsin Southeast Asia in the 14th century.

To get a better understanding of Angkor's urban landscape, Fletcher's colleague at Sydney, Damian Evans, turned to LiDAR, an instrument that a few years ago mapped hidden features of medieval Mayan ruins in Central America. Using a helicopter for just 20 hours of flight time in April 2012, a consortium put together by Evans imaged 370 square kilometers of terrain, encompassing Angkor and two nearby temple complexes, Phnom Kulen and Koh Ker. LiDAR laid bare the imprints of a 9th century city on the Kulen, known from inscriptions as Mahendraparvata. "We found the great early capital of the Khmer empire," Fletcher says. "The discovery of this early Angkorian city is a very exciting example of LiDAR's use in the region," adds Miriam Stark, an anthropologist at the University of Hawaii, Manoa, who has recently started to conduct research at Angkor but was not involved in last year's LiDAR campaign.

The LiDAR survey also showed that the medieval settlements at Phnom Kulen and Koh Ker had extensive hydraulic engineering on a scale comparable to Angkor, showing a much wider reliance on water management systems "to ameliorate annual-scale variation in monsoon rains and ensure food security," the team reports. The LiDAR readings also uncovered cryptic coil-shaped rectilinear embankments covering several hectares near Angkor Wat. "It was an unbelievable surprise," Fletcher says. "Nothing like them had been seen before in Khmer architecture." He speculates that they had some role in farming, but for now their function remains a mystery.

The LiDAR data also add weight to the idea that Angkor's complicated waterworks unraveled. It uncovered "very serious" erosion in parts of the ancient city that, Fletcher believes, accounts for the deep sand deposits documented in the team's excavations.

*This story provided by ScienceNOW, the daily online news service of the journal *Science.