The great medieval temple of Angkor Wat in Cambodia was once at the centre of a sprawling urban settlement, according to a new, detailed map of the area.
Using radar, an international team have discovered at least 74 new temples and complex irrigation systems.
The map, published in the journal PNAS, extends the known settlement by 1000 sq km, about the size of Los Angeles.
Analysis also lends weight to the theory that Angkor's residents were architects of the city's demise.
"The large-scale city engineered its own downfall by disrupting its local environment by expanding continuously into the surrounding forests," said Damian Evans of the University of Sydney and one of the authors of the paper and map.
Working with researchers from Australia, Cambodia and France, the map was produced from ground surveys, airborne photography, and ground-sensing radar from Nasa's AIRSAR flying laboratory.
"The radar can sense differences in plant growth and moisture content that result from topographical variations of less than a metre," Mr Evans said.
The data allowed the researchers to peer through the vegetation that now shrouds the World Heritage site.
It suggests that the medieval settlement surrounding Angkor, the one-time capital of the Khmer empire which flourished between the ninth and 14th centuries, was at least three times larger than previously thought.
The team believes it could have covered 3,000 sq km (1,150 sq miles), the largest pre-industrial complex of its kind.
Its nearest rival is Tikal, a Mayan city in Guatemala, which covers between 100 and 150 sq km (40-60 sq miles).
The detailed survey also allowed the researchers to map at least 74 new temples as well as more than 1,000 manmade ponds.
They also discovered that the city's water supply probably relied on a single complex channel that extended 20 to 25km out from Angkor city.
The researchers say that the system, until now thought to be purely decorative and ceremonial, was probably used to support farming, in particular intensive rice agriculture.
In all, the newly mapped terrain could have supported half a million people, the researchers believe.
The new analysis of the irrigation system also sheds light on the civilization's collapse in the 14th century.
"We saw signs that embankments had been breached and of ad hoc repairs to bridges and dams, suggesting that the system became unmanageable over time," Mr Evans told the AFP news agency.
In addition, deforestation, over population, topsoil erosion could have contributed to the population's sudden disappearance.
"Angkor was extensive enough, and the agricultural exploitation intensive enough, to have created a number of very serious environmental problems," he said.