But first there arose Phnom Kulen, the birthplace of the great Khmer civilization that dominated most of Southeast Asia from the 9th to the 15th centuries. Phnom Kulen is only some 25 miles north of a metropolis that reached its zenith three centuries later-the greatest city of the Khmer Empire, and possibly the most glorious religious center in the history of mankind: Angkor, derived from the Sanskrit word nagara, or holy city, site of the famed temple Angkor Wat. “But this gave us the evidence we were hoping for.”

“We knew this might be out there,” says Chevance, as we roar back down a jungle trail toward his house in a rural village on the plateau. Numerous excavations as well as high-tech laser surveys conducted from helicopters have revealed that the lost city was far more sophisticated than anyone had ever imagined-a sprawling network of temples, palaces, ordinary dwellings and waterworks infrastructure. It lies on this overgrown 1,300-foot plateau, known as Phnom Kulen (Mountain of the Lychee fruit), northeast of Siem Reap. The vast urban center that Chevance is now exploring was first described more than a century ago, but it had been lost to the jungle until researchers led by him and an Australian colleague, Damian Evans, rediscovered it in 2012. It’s proof of an advanced engineering system that propelled and sustained a vanished civilization. Yet this isn’t merely an overgrown pond we’ve stumbled into. We are quite possibly the first human beings to set foot in this square-shaped, man-made reservoir in more than 1,000 years. Chevance, a lanky 41-year-old dressed in olive drab and toting a black backpack, smiles triumphantly. Seconds later, as if on cue, the ground beneath our feet gives way, and we sink into a three-foot-deep muddy pool.

“That’s an indication of a permanent water source.” “You can see that the vegetation here is very green, and the plants are different from the ones we have seen,” he says. We’ve ripped our arms and faces on six-foot shrubs studded with thorns, been savaged by red biting ants, and stumbled over vines that stretch at ankle height across the forest floor. For two hours, Chevance, known to everyone as JB, has been leading me, along with a two-man Cambodian research team, on a grueling trek. The temperature is pushing 95, and the equatorial sun beats down through the forest canopy. Paused in a jungle clearing in northwestern Cambodia, the French archaeologist studies his GPS and mops the sweat from his forehead with a bandanna. Jean-Baptiste Chevance senses that we’re closing in on our target.

Lonely tower medieval ruin forest