“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.