The "lost city of Angkor" was painstakingly uncovered by French archaeologists who spent much of last century rescuing it from the forest and restoring it to its former splendour.
Not surprisingly, they concentrated their efforts on the massive temples, which were built between the ninth and 13th centuries as monuments to the power and wealth of the Khmer kings.
The rest of the region remained carpeted with vegetation, with few remnants of the ancient civilisation visible to the human eye.
A French, Cambodian and Australian team used aerial photographs, satellite imagery and high-resolution ground-sensing radar provided by Nasa to investigate what lay beneath the green cloak.
What they found was the remains of 74 temples, as well as the sites of thousands of houses, roads, embankments, canals and ponds - all believed to have been part of an extensive residential complex that included a large system of waterways.
The team has just published its findings, together with a detailed map, in the American journal Proceedings of the National Academy of Sciences.
"People never really considered Angkor as being much more than a scattering of temples in the landscape," said Damian Evans, an Australian archaeologist who worked on the project. "In fact, it would have been a huge and popular city, full of life."
He and his colleagues report that "even on a conservative estimate, greater Angkor at its peak was the world's most extensive pre-industrial low density urban complex" - far larger than the ancient Mayan cities of Central America, for instance, which covered 260 square kilometres at most.
Mr Evans, based at the University of Sydney's Archaeological Computing Library, said that the Khmers of 1000 years ago appear to have lived very similar lives to modern-day Cambodians.
"They lived in clusters of houses on raised mounds to keep above the floodwaters in the wet season," he said. "The mounds were in clusters, and scattered through them were these small village ponds.
"Between the houses were rice fields. And the core of this system was the village temple, in much the same way that Buddhist temples are the core of contemporary Cambodian communities."
The Khmer people subsisted on rice agriculture, just as many Cambodians still do, and it is believed that the water management system - designed to trap water coming down the hills in the north - was partly used for irrigation.
The newly discovered temples, cautions Mr Evans, were not grand like those at the heart of Angkor.
Most now consist only of a pile of brick rubble, plus the occasional sandstone door frame or pedestal.
But while they hold little interest for tourists, they are valuable archaeological finds because they were centres of taxation, education and water control, as well as religion.
A succession of Khmer kings ruled the Angkor area from about 800AD, producing the architectural masterpieces and sculpture now preserved within a World Heritage-listed area.
By the 13th century the civilisation was in decline, and most of Angkor was abandoned by the early 15th century, apart from Angkor Wat, the main temple, which remained a Buddhist shrine.
When the lost city was rediscovered, archaeologists were absorbed by the need to rescue and conserve the dozen or so main temples and their extraordinary bas-relief carvings. Few excavations were carried out outside the temple precinct.
"No one really thought to look beyond them and into the broader landscape, to see how people actually lived," Mr Evans said.
By the 1960s it was clear that rich archaeological pickings lay beyond the walled city. A programme was put in place to investigate the wider area, but never got off the ground because of civil war, followed by the advent of the Khmer Rouge and Pol Pot's murderous regime.
It was not until the 1990s that the security situation improved, enabling work to resume.
But when the international mapping team started their project, they still needed an armed escort for protection when they visited certain areas.
And Evans says that even now he never steps off marked paths, because of the risk posed by unexploded landmines.
The settlement mapped by the team existed from 500 to 1500AD, and could have supported a population of up to one million people.
"Now we have the map, we can quantify this residential space," Mr Evans said. "We can start to do proper demographic studies and work out how many people were living on these mounds.
"But we can say now, from a preliminary point of view, that it would have had a population of several hundred thousand, at least."
The city was criss-crossed by roadways and canals, and was similar to modern cities that suffer from urban sprawl.
The team may also have found the key to Angkor's collapse - or, least, confirmed an existing theory: that the city "built itself out of existence".
"The water management system, in particular, had the potential to create some very serious environmental problems and radically remodel the landscape," Mr Evans said.
"Angkor would have suffered from the same problems as contemporary low-density cities, in terms of pressure on the infrastructure, and poor management of natural resources like water. But they had limited technology to deal with these problems and failed to ultimately, perhaps."
Excavations in the next few years will examine the theory in more detail.
The new archaeological evidence will pose a challenge for conservationists, as the current World Heritage site covers 388 square kilometres, which are intensively managed and protected.
Cambodian authorities, meanwhile, are grappling with the problem of how to preserve the precious ruins within the temple precinct from increasing numbers of visitors.
The government is expecting three million visitors in 2010, and many of those will head to the temples. Angkor Wat is now one of Southeast Asia's leading tourist attractions.
Soeung Kong, deputy director-general of the authority which oversees Angkor, told Agence France-Presse recently: "The harm to the temples is unavoidable when many people walk in and out of them. We are trying to keep that harm at a minimal level."
The main problem, however, lies in Siem Reap, the nearby town that has mushroomed in recent years to accommodate the growing numbers of tourists.
There are now more than 250 guesthouses and hotels, which have been sucking up groundwater and destabilised the earth beneath the Angkor complex.
At least one monument, the Bayon temple - famous for the serene faces carved on its 54 towers - is collapsing into the sandy ground.
VITAL STATISTICS
* 1800sq km - a similar size to Greater London.
* 74 temples.
* Thousands of houses, roads, embankments, canals and ponds.
* Existed from 500AD to 1500AD.
* Could have supported a population of up to one million people.
- INDEPENDENT
