The experts spent years studying NASA images of the Angkor region and checking possible sightings on the ground, and found enough ruins to conclude that the site was the largest settlement in the pre- industrial world.

Carpeted with vegetation and obscured by low–lying cloud, the ruins spill over about 400 square miles around the distinctive temple, and are linked by a complex irrigation system.

The findings could pose a problem for conservation experts, as the historical remains spread far beyond the designated World Heritage site around the temple.

Damian Evans of the archaeological computing laboratory at the University of Sydney, and colleagues from Australia, Cambodia, and France combined information from hand-drawn maps, ground surveys, airborne photography and radar provided by NASA. The radar can sense differences in plant growth and moisture content produced by small changes in surface structure or height. The team found evidence of more than 1,000 new man-made ponds and at least 74 ruined temples.

One single hydraulic system linked the entire network and was probably used to provide citizens with a stable water supply through the unpredictable monsoon season. They also discovered two mysterious giant earthen structures.

The new map was published in the journal Proceedings of the National Academy of Sciences.

The experts said: ‘’Even on a conservative estimate, greater Angkor at its peak was the world’s most extensive pre-industrial low- density urban complex.’’

The city stood from the ninth to the 16th centuries, when it collapsed. The team said there were signs that its citizens helped to engineer their own downfall by disrupting the local environment.

‘’Angkor stands in a vast expanse of rice fields that would have required extensive forest clearance over the entire Angkor plain and up to the Kulen and Khror hills to the north,’’ the experts said. ‘’The new maps show that land use modification at Angkor was both extensive and substantial enough to have produced a number of very serious ecological problems, including deforestation, overpopulation, topsoil degradation and erosion.’’