Sprawling Angkor Brought Down by Overpopulation, Study Suggests

Susan Brown for National Geographic News

August 13, 2007

Cambodia's long-lost temple complex of Angkor is the world's largest known preindustrial settlement, reveals a new radar study that found 74 new temples and more than a thousand manmade ponds at the site. (See a photo gallery of Angkor's newly uncovered sprawl). But urban sprawl and its associated environmental devastation may have led to the collapse of the kingdom, which includes the renowned temple of Angkor Wat, the study suggests. Ever since the late 16th century, when Portuguese traders spied the towers of the monument poking through a dense canopy of trees, people have puzzled over the demise of the Angkor civilization. Now a new archaeological map created using jungle-penetrating radar has revealed traces of vast suburban sprawl surrounding the many temples and the walled central city of Angkor Thom. Extensive waterworks threaded through the low-density development, channeling the flow of three rivers through agricultural fields, homes, and local temples. In the end, residents of greater Angkor likely struggled with the ecological consequences of transforming the landscape. The new survey found breached spillways and canals clogged with silt, suggesting that environmental degradation made the infrastructure increasingly difficult to maintain. Mystery Civilization The Khmer kingdom of Angkor rose in the ninth century A.D. and thrived for 600 years before its leaders left to resettle near the modern Cambodian capital of Phnom Penh. The civilization is renowned to this day for building the massive temple of Angkor Wat—often called the world's largest single religious monument—in the early 12th century. Although the kings of Angkor etched into stone such significant moments in their history, they left no message during the settlement's decline. That mysterious departure has fueled a smoldering controversy among archaeologists. Until recently, warfare and changing religion were the prime suspects, according to anthropologist Charles Higham of the University of Otago in Dunedin, New Zealand. Higham's own work has unearthed evidence into the origins of Angkor. 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. Sprawling City 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. Unsustainable Development 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.