The megacity of Angkor was once the capital of the Khmer nation, an empire which flourished from approximately the 9th to the 15th century. With a population near 1 million and a size of roughly 1,000 square kilometers, Greater Angkor was the largest pre-industrial metropolis in the world. Located in modern-day Cambodia, Angkor is not only a UNESCO World Heritage Site visited by two million tourists per year, but also a historical case of the growth and demise of a human-made urban complex exposed to severe climatic instability. Roland Fletcher is a Professor of Theoretical and World Archaeology at the University of Sydney and the Director of the Greater Angkor Project, an international collaboration which has been studying the urban landscape since 2002. Combining ground surveys, tree-ring dating, and excavations with technologies such as airborne laser imaging (Lidar), Fletcher and his group have gained new insights into the development of Greater Angkor and the risks it faced. Their studies have shown that the extensive, low-density settlement cleared its landscape of natural vegetation for agricultural purposes and was dependent on a massive infrastructure – conditions which made it vulnerable to extreme climatic instability. Roland Fletcher shows how these conditions have resonance in the 21st century – at a time when megacities and climate change are more prevalent than ever before in human history.