Invaders from Thailand sacked central Angkor Tom in 1453, Higham noted. And religious affiliations shifted from a form of Buddhism that recognized earthly deities to one that did not.
"The great lumpen proletariat ceased to believe in the divinity of the king, another reason the social fabric of Angkor began to change," Higham said.
Another possibility is that overexploitation, overpopulation, and deforestation overwhelmed a fragile monsoon habitat, Higham added.
Archaeologist Bernard-Phillipe Groslier of the French Research School of the Far East (EFEO) first proposed the environmental-downfall theory nearly 30 years ago.
But his institution's efforts concentrated on preserving the fallen monuments, and Cambodia's then civil war—and the grim reign of the Khmer Rouge—shuttered the school.
For decades conflict and strife precluded work in the area. Even now, land mines litter the northern reaches of Angkor.
But in 1992 archaeologist Christophe Pottier reopened EFEO and began surveying the surrounding region by motorbike, using aerial photographs and earlier radar images to guide his exploration.
Most recently the Greater Angkor Project commissioned a series of radar images from NASA with much finer resolution, which archaeologist Damian Evans of the University of Sydney combined with maps Pottier had compiled.
The new archaeological map reveals medieval suburbs spreading from the shores of Cambodia's Tonle Sap to the Kulen highlands to the north (see a map of Cambodia).
At more than 3,000 square kilometers (1,160 square miles) it is the largest settlement ever found from the preindustrial world, the authors say.
"The scale is really unlike anything else," said archaeologist William Saturno of Boston University, noting that it dwarfs the large Mesoamerican cities that are the subject of his own work. (Related: "Oldest Intact Maya Mural Found in Guatemala" [March 22, 2002].)
The people who built greater Angkor were accomplished civil engineers, he added.
"It's amazing the amount of earth they moved."
Rectangular embankments enclosed artificial ponds and probable rice paddies, while earthen mounds lifted houses to avoid seasonal floods.
The waterworks also diverted water from the Puok, Roluos, and Siem Reap rivers to reservoirs that could be drained to irrigate crops or filled to dampen extreme flooding, Evans's team reports in the Proceedings of the National Academy this week.
The scientists also found evidence that the system had become overwhelmed by the time Angkor met its downfall.
A breached spillway, for example, was filled with blocks from walls that had tumbled down or were possibly pushed into the channel and buried in sand. And thick layers of sediment dropped by churning floodwaters fill some of the canals.
"Something went terribly wrong," Evans said.
As Angkor's growing population expanded, they must have cleared forests for agricultural fields, which may have led to erosion and flooding, the study authors speculate.
But no one can say for sure whether the collapse of the waterworks helped precipitate the Khmers' departure or followed the site's abandonment and neglect.
As land mines are cleared from the region, archaeologists are moving in to excavate the structures and date pollen grains found within the sediments in an attempt to solve the puzzle.
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