Enlarge / The Cambodian city of Angkor was once the largest in the world until vast swathes of the population decamped in the 15th century. Its famous temple, Angkor Wat (above), survived. (credit: Stefan Irvine/LightRocket/Getty Images) The Cambodian city of Angkor was once the largest in the world... then the vast majority of its inhabitants suddenly decamped in the 15th century to a region near the modern city of Phnom Penh.