In the early Middle Ages, nearly one out of every thousand people in the world lived in Angkor, the sprawling capital of the Khmer Empire in present-day Cambodia. But by the 1500s, Angkor had been mostly abandoned—its temples, citadels, and complex irrigation network left to overgrowth and ruin. Recent studies have blamed a period of unstable climate in which heavy floods followed lengthy droughts, which broke down the infrastructure that moved water around the massive city.

But it turns out Angkor’s waterworks may have been vulnerable to these changes because there was no one left to maintain and repair them. A new study suggests that Khmer rulers, religious officials, and city administrators had been steadily flowing out of Angkor to other cities for at least a century before the end.

A long road to ruin

University of Sydney environmental historian Dan Penny and his colleagues took sediment cores from a moat near the south gate of Angkor Thom, the citadel at the administrative and political heart of the city and the Khmer Empire. Year after year, windblown sediment and runoff from the city’s drainage system settled to the bottom of the moat, storing pollen from local crops, particles of charcoal from burning, and sediment from cleared land. It makes a good measure of activity in the city: the more Angkor’s administrators cleared land, built new structures, and otherwise disturbed the landscape, the more sediment washed and blew into the moat.

In the late 11th or 12th century CE, when the Khmer dug the Angkor Thom moat, pollen trapped in the sediment revealed a mixture of forest and rice farms surrounding the city, and traces of charcoal suggested regular burning to clear land for planting. But in the early 1300s, the amount of pollen from domestic crops started a steady decline, as did the traces of charcoal. Around the same time, the amount of sediment washing into the moat suggested that land use around Angkor Thom was slacking off and that the city’s irrigation and drainage system wasn’t being maintained.

By the mid-1300s, a few decades after the first signs of trouble, the top layers of the sediment core are mostly thick peat, suggesting a floating mat of swamp plants clogging the once well-maintained moat. That paints a grim picture of the state of the city. But how did it happen?

“Most of the visible damage done to Angkor’s urban infrastructure occurred as a result of flooding in the latter half of the 14th century,” Penny told Ars. But the sediment cores suggest that Angkor had been in trouble for decades before the floods, and that lines up well with archaeological evidence; the last religious monument built at Angkor dates to 1295 CE. The sediment cores start to record a drop-off in land clearing and infrastructure work about twenty years after that.

“It is tempting to see the lack of ostentatious building works as further evidence of a decline in Angkor’s administrative and political functions,” Penny said. “This adds a new level of complexity to the ‘collapse’ of Angkor—did the elite leave because the city failed or was invaded, or did the city fall because the elite were already busily engaged elsewhere?”

An absent ruling class

At first glance, it’s easy to imagine Angkor’s residents abandoning the city in a single catastrophe, perhaps when the floods of the mid-1300s devastated its water distribution network or when foreign armies sacked the city. But the layers of sediment in the Angkor Thom moat, along with historical sources, suggest this wasn't what happened. Instead, the ancient capital slowly slid into disrepair as its rulers and administrators trickled out of Angkor toward new centers of power in the Mekong Delta and along the Tonte Sap River.

A complicated set of factors led to Angkor's abandonment, but Penny and his colleagues suggest that one of them has to do with infrastructure again—the city may simply have outgrown itself.

“By the end of the 12th century Angkor was effectively ‘full’—everything that could be developed had been developed,” Penny told Ars. “Large infrastructure like a temple or a reservoir is difficult to move or modify, which limits options for new types of construction and effectively restricts options to adapt to change. As large infrastructure networks begin to fail but are too complex to maintain and too large to avoid, adaptive options like relocation become viable.”

In other words, the people responsible for maintaining Angkor’s aging infrastructure may have simply left because the city’s design made the task of doing so too difficult, especially when they were drawn elsewhere by the lure of Indian Ocean trade and the need to counter increasingly restless neighboring powers. That effort eventually failed when the Ayatthaya Kingdom (in modern-day Thailand) invaded Angkor in 1431, sounding the official death knell of the former Khmer capital.

“Resilience is the key,” Penny said. “We have found that overly complicated and interdependent networks create a vulnerability to change in large, low-density urban environments.”

After the end

But even as the city crumbled from the inside, the outlying farmland and small communities in Angkor's shadow seem to have kept right on going—and may even have thrived.

“The wealth of Angkor’s elite comes from an elaborate system of tribute and taxation that operated through a network of temples,” Penny told Ars. So the gradual departure of religious and political elites may have lifted an economic burden. “Further, production in many areas of greater Angkor is more dependent upon resilient environmental resources such as ground water and the seasonal flood of the Mekong River than it is upon the patronage of the king or the elite,” Penny said.

And even as the religious and administrative class gradually left Angkor behind, the city’s other residents hung on despite the crumbling infrastructure. The Thai occupiers stayed only about ten or twelve years, but a sparse scattering of archaeological traces—bits of crops eaten, sculptures, and ceramic vessels—suggest that people still lived in the former capital and sometimes modified existing buildings for their own use.

“While the data are fragmentary, it is widely accepted that parts of Angkor, particularly important temples like Angkor Wat, were never completely abandoned and that a residual population persisted in many areas,” Penny said. “How large that population was, who they were, and how they lived remains a fascinating question.”

In the years after the fall of Angkor, a series of Cambodian regimes have constructed their own stories about what befell the city and the Khmer Empire. Many versions of the story focus on “loss at the hands of interventionist neighboring states,” as Penny and his colleagues put it.

PNAS, 2018. DOI: 10.1073/pnas.1821460116;(About DOIs).