Between the 9th and 13th centuries the kings of the Khmer Empire built, around their successive capitals north of the Tonlé Sap lake, the largest settlement complex of the pre-industrial world. Airborne laser scanning (lidar) since 2012 has made the old guesses concrete: Greater Angkor was a low-density urban sprawl of temples, house-mounds, roads, and canals covering on the order of 1,000 square kilometres, with a population plausibly approaching a million.
Its engine was water. Enormous reservoirs (barays) — the West Baray alone is 8 km long — linked by hundreds of kilometres of canals and embankments, caught the monsoon and banked it against the dry season, watering rice systems that could feed a city that size. Its monuments still state the scale: Angkor Wat, raised by Suryavarman II in the early 12th century, remains the largest religious structure on Earth; the Bayon’s stone faces and the flooded logistics of Jayavarman VII’s hospitals and rest-houses mark the empire’s zenith around 1200.
What went wrong
The traditional story gave Angkor a clean death: sacked by the Siamese kingdom of Ayutthaya in 1431, abandoned, swallowed by jungle, “discovered” by the Frenchman Henri Mouhot in 1860. Almost every clause of that needs correcting. The sack happened, and the court did shift south toward the Phnom Penh region in the mid-15th century — closer, not incidentally, to the maritime trade with China that was becoming the region’s real money. But tree-ring records from Vietnamese cypresses, read against the archaeology, show the deeper churn: decades-scale megadroughts in the 14th-15th centuries, punctuated by violently wet monsoon years whose floods tore through the water network — a system so vast, interdependent, and silted with age that repair may have been beyond any administration. Excavated canals show exactly that: choked intakes, breach deposits, failed rerouting. The city’s greatest asset had become its greatest liability — infrastructure too big to fix.
Religious change rode alongside: the god-king temple economy of Angkor’s Hinduism and Mahayana gave way to Theravada Buddhism’s leaner institutions, dissolving the labour machine that temple cities ran on.
Never lost
And “lost” was always the wrong word. Angkor Wat was never abandoned — converted to Theravada use, maintained by monks, visited by Asian pilgrims and the odd Portuguese friar centuries before Mouhot, whose posthumous bestseller merely introduced Angkor to European readers (he himself credited earlier accounts). Sixteenth-century Khmer kings even restored parts of it. The lidar surveys that mapped the medieval sprawl found something else, too: the countryside never emptied — people stayed among the rice fields as the court and the stone-building stopped.
Angkor today anchors Cambodian identity — it is on the national flag — and a tourism economy its water engineers would have understood: the modern city of Siem Reap now pumps so much groundwater that engineers worry about the monuments subsiding. The city that drowned in its own hydraulics is being undermined by thirst.