Cambodia's famed Angkor—usually penciled into guidebooks thanks to its eponymous 12th-century temple—was once the world's biggest city, new research by University of Sydney archaeologists shows. From the 10th century on, Angkor grew to nearly one million inhabitants and sprawled out to the size of modern-day Los Angeles.
"The scale is truly unparalleled," says one digger. But by the 16th century, the ambitious water supply system that caused the city to boom also helped hasten its downfall. As Angkor expanded to include over 1000 ponds, 74 temples, and over 115 sq. mi. of development, a combination of overpopulation and deforestation caused the canals to silt up.