via Livescience, a new satellite study of the ancient Cambodian site of Angkor Wat determines that not only was it once a city rather than just the site of a religious temple complex, but that it might have been one of the world's largest cities in terms of area, covering nearly 400 square miles. The site also had a sophisticated system of irrigation ponds running throughout the area and that the system might have contributed to the city's downfall.
"Angkor was surrounded by a vast expanse of rice fields that would have required extensive forest clearance. Over time, the intense farming could have led to serious ecological problems, including those associated with deforestation, overpopulation, topsoil degradation and erosion."
Angkor Wat was the capital city of the Khmer Empire from the 9th to the 16th centuries located in modern Cambodia. The city might have held several hundred thousand citizens at its height.