'It should be about here," Stéphane De Greef says. He glances at a hand-held GPS, then examines his surroundings: charred tree stumps, termite mounds and a low tangle of bushes, saplings and thorny vines that cling to clothing and scrape flesh.
On the GPS screen, a black-and-white image shows that he is standing where two straight lines converge – the intersection of two highways that are 50 metres wide, an ancient crossroad now obscured by an abandoned jungle farm.
"There was a city under our feet," the Belgian cartographer says, "but for so long, we were lacking the tools to see it."
Lessons from the past
In June, an international team of researchers announced the discovery of a 1,200-year-old city cloaked in the forests of northern Cambodia. It was the size of the country's modern capital and so sophisticated that the lead scientist feels it may even produce a better understanding of modern cities.
Abandoned to the jungle for a millennium, Mahendraparvata (Mountain of the Great Indra) was located – with the help of a Canadian engineering firm – atop Phnom Kulen, a sparsely populated plateau just 40 kilometres northeast of Angkor Wat, the famed temple complex that is Cambodia's greatest tourist attraction (it drew more than two million visitors last year).
To contemporary Cambodians, Phnom Kulen is a place of living myth. It was here that, in 802, the legendary King Jayavarman II declared himself the divinely sanctioned ruler of a newly unified and independent Cambodia. This gave rise to the Khmer Empire – a civilization that would build the ancient city of Angkor, the largest preindustrial settlement in the world, and dominate the region for the next 600 years.
Scholars have long known of Mahendraparvata's existence. Working from 10th- and 11th-century Sanskrit inscriptions found elsewhere, French teams began to traverse Phnom Kulen in the 1880s, hoping to find traces of what had been one of Jayavarman II's capitals. They discovered dozens of overgrown and crumbling Hindu temples atop the plateau, but few signs of earlier habitation.
With academic attention focused on the grander monuments at nearby Angkor, their work remained obscure, and the hunt for Mahendraparvata came to a halt in the late 1960s with the outbreak of civil war.
For more than two decades, Cambodia remained closed as it was governed by succession of authoritarian regimes. The bloodiest of these, the infamous Khmer Rouge of Pol Pot, maintained a presence at Phnom Kulen until 1996 (17 years after being driven from power), leaving behind a cruel legacy of anti-personnel mines and amputees.
Four years later, French archeologist Jean-Baptiste Chevance began to do research in the area. In 2008, he created the Archeology and Development Fund (ADF), the area's only research outfit. It also invests in development projects, which help the 4,000 people living atop the sprawling plateau today, most of whom survive by slash-and-burn farming, illegal logging and poaching.
"Phnom Kulen is isolated and poorly studied. It's uncomfortable working here and dangerous too – I just love it," Mr. Chevance says.
A guiding light
Finding a city buried in the jungle requires more than passion.
Damian Evans, a Victoria native who came to Cambodia as an undergraduate researcher and now leads the University of Sydney's archeology centre at Siem Reap, had been studying the area for years. "The first 10 years of my work here was focused on using aerial photographs and radar to map Angkor's extended agro-urban periphery, but there were always these big blank spaces in the middle of my maps where forests stood," he says.
Then he was approached by the Indonesian arm of Vancouver-based engineering, surveying and mapping firm McElhanney. The company offered him the use of lidar, a laser tool that succeeded in uncovering archeological sites in Central America, at cost.
Similar to radar, lidar can measure subtle differences in surface elevation by emitting hundreds of thousands of laser pulses per second, then calculating the time it takes them to return. When combined with GPS technology, lidar data can be used to create detailed topographical maps.
"Lidar is revolutionizing the archeology of tropical forest environments," Mr. Evans says. "If light can pass through gaps in the leaves and hit the forest floor, we can get accurate maps of the ground. When you have things such as mounds and depressions just a metre or two beneath the landscape, they are reflected in the surface topography – and it is those surface expressions that lidar picks up."
Mr. Evans decided to try mapping the ephemeral city of Koh Ker first. To mitigate the huge cost of mobilizing the lidar equipment, he joined the Cambodian government to form a consortium of eight archeological and cultural instructions, including Mr. Chevance's ADF.
Over two weeks in April, 2012, the million-dollar lidar system was mounted on a helicopter, then flown in parallel lines at several sites, covering a total of 370 square kilometres at a cost of about $250,000 (U.S.).
The consortium's findings were published in Proceedings of the National Academy of Sciences of the United States of America in July. The lidar survey helped to uncover more of ancient Angkor's massive reach, but its principal revelation was Phnom Kulen's "lost" city.
"At first, I couldn't believe it," Mr. Chevance says. "While we always suspected that something was there, the complexity of it was astounding. Lidar linked the monuments we knew of, to reveal a complete preplanned urban network."
Temple or termite hill?
Stéphane De Greef, the cartographer, pulls up a three-dimensional map on his laptop. "Everything we knew about this place before is in green, and all this red stuff shows new sites identified since then," he says. "As you can see, we had missed 90 per cent of it."
The city, which sports a complex hydrological system of dikes, canals and reservoirs, is aligned on a grid. Its largest highway is 60 metres wide and eight kilometres long.
Mr. De Greef zooms in to show a square enclosure with a central mound: a newly identified temple. Roads connect it to other such sites, as well as rectangular housing plots with ponds. In the centre of the city stands a massive pyramid.
"Besides temples and roads, lidar revealed strange structures, like an ordered field of large, earthen mounds and a series of interconnected ponds," he says, having analyzed the Phnom Kulen data for more than a year. "Their functions are still a mystery to us."
To better understand the city revealed by lidar, he, Mr. Chevance and Cambodian archeologist Sakada Sakhoeun have spent the past six months conducting ground-verification missions. That is, dodging ox carts and battered motorcycles, they have been racing along Phnom Kulen's forested trails on motocross bikes, then trekking with machetes to investigate every bump, line and depression displayed on their maps.
On one such mission, Mr. De Greef stops his bike at a fork in the muddy, forested track. Picking up a handful of earth, he sifts through it to reveal shards of ancient pottery and broken roof tiles.
"The people living here know that this was an important place," he says, "but I don't think anyone realized that it was a city the size of Phnom Penh."
GPS in one hand and machete in the other, he continues into the jungle, hacking at the underbrush until he arrives at a small pile of rust-coloured brick half-buried in mud and coated with vegetation: one of the 30 temples discovered by the ADF team.
"Sometimes, what look like temples on the screen turn out to just be termite hills," Mr. De Greef says. "Until you get into the field, you never really know what you're going to find."
Although none of the newly discovered temples is intact, the ADF team has been loath to share its maps. "Most every temple in Cambodia has already been looted," Mr. Chevance says, "but a few of the temples we've found with lidar are still untouched. We don't want people coming up here and jeopardizing our work."
According to Mr. Evans, by exhuming an ancient city, scientists may ultimately produce a more nuanced understanding of modern urbanism.
"Huge, low-density cities like Angkor were relatively unusual in the medieval world, but have become a defining characteristic of contemporary urban life," he explains. "Over time, the populations who inhabited these early Cambodian cities found themselves increasingly constrained by a legacy of centuries of massive infrastructural development. It seems to have created a kind of inertia that limited their capacity to adapt to changing environmental circumstances."
In the case of Angkor, it is believed that the eventual failure of the massive city's canals and reservoirs – a complex yet rigid hydrological system that probably supported a population of nearly one million – may have hastened the Khmer Empire's demise.
"Ultimately, our objective is to see what kind of lessons can be learned that are of broader relevance to contemporary societies," Mr. Evans says.
At Phnom Kulen, meanwhile, the team labours on. Three more months are needed to finish ground verifications, and excavations have only just begun.
Moreover, with ancient roads and highways stretching to the very edges of the 30 square kilometres surveyed by McElhanney, Mr. Chevance is trying to raise money for a second lidar campaign next year to determine the true size of the long-hidden city and to see what other mysteries may be concealed by the forests of Phnom Kulen.
"I believe Mahendraparvata is much bigger than the window we're glimpsing," he says. "We would like to use lidar to survey five times what we've already covered. If all goes well, we'll be working up here for years to come."
Lidar at a glance
Lidar (light detection and ranging), which was developed in the 1960s, measures distance by illuminating a target with a laser and analyzing the reflected light.
First used in meteorology.
Used to map the moon's surface in 1971.
Used by an archeologist for the first time in Costa Rica in 1988.
Became a powerful surveying tool with the advent of reliable GPS systems in the 1990s.
Used extensively over the past five years to map archeological sites in Mesoamerica.
Cambodian survey was the first of its kind in Asia and the largest archeological lidar survey ever conducted in the world.
Commonly used today in forestry, mining, geology, surveying, coastal management, etc.