On the back wall of the classroom at Sapa O'Chau, a bootstrap operation in Sapa town, far northern Vietnam, where hill tribe children study to be tour guides, colored-pencil drawings depict young girls with tears streaming down their faces. Some are shackled with metal cuffs; others are trapped in cages or giant jars. The most common scene shows a girl in a forest, trailing a male figure grabbing her by the wrist. "They may pretend to be your friend so they can take you away," a tiny scrawl reads. "You must be very careful."

The students drew the pictures in May 2012, shortly before participating in a made-for-TV documentary by MTV Exit, an initiative that campaigns to end human trafficking. At one point during the program, the members of Canadian pop-punk band Simple Plan sit in a circle with the kids and ask if any of them knows someone who has been trafficked. One girl, Ly, raises her hand. About a year ago, she says, her cousin boarded the motorbike of a handsome boy whom she trusted. No one has seen her since.

"I dream of her a lot," Ly says in front of the camera.

I watched the video with Sapa O'Chau's then-general manager, Peter Gilbert, one evening at the organization's shophouse office in town. Onscreen, none of the other students volunteered an answer. But three of their own classmates had vanished down the mountain. One girl had been taken in the same manner as Ly's cousin. The other two, also girls, had gone on their own. They had wanted to be tour guides, but their lack of English made this unlikely. "I think they felt life would be tough here, and they didn't see much hope," Gilbert said. "I guess they decided to go together, or maybe one first made that decision and then worked on the other until she agreed as well. And then they just disappeared."

The girls at Sapa O'Chau draw anti-human-trafficking pictures | (Philip Jacobson/Courtesy Latterly)

Outside on the veranda, Gilbert smoked a cigarette as I asked how the kidnappings worked. He stressed that he couldn't be sure — no one I talked to is sure — but he ventured that it was usually someone the girl knows: a boy she meets, maybe one who has a nice motorbike, nice clothes, who takes her shopping, tells her nice things. The girl falls in love, comes to trust the boy.

"Then one day, maybe she gets on that motorbike, just for a little ride around the lake," Gilbert said. "But suddenly he drives her miles away, and it's not long before she's lost, and she can't get off the bike because she'll hurt herself. The girl gets threatened, the boy takes her phone; maybe he takes her somewhere where it's not just one boy but a group of them. And all of a sudden she's helpless, trapped, captured.

"Then it seems to be they end up in a brothel, or married, forced marriage. I've heard a story that the girls prefer the brothel because it's probably closer to the border, so it's easier for them to get away; whereas, if they were married it's probably thousands of miles away and they could disappear into the interior of China."

China — that's where they go, anyone in Sapa will tell you. The country is desperately bereft of women, the result of a cultural preference for boys amid the one-child policy. China shares a long, porous border with Vietnam across which traffickers can easily spirit girls like Ly's cousin. They pluck them from all over the region, luring or simply seizing them with a range of methods, from pretend romances to promises of employment to forcing them in a car and driving off.

If trafficking happens in pockets, though, Sapa is unique, for in few places is the world changing so quickly as at this outpost of development in the Himalayas' eastern extremities, the gateway to northern Vietnam's hill tribe communities. While striking in variety and interest, not least for their famously vibrant traditional forms of dress, these groups are by and large impoverished, uneducated, and disconnected from the protections of the state, heightening their vulnerability to predators. The Black Hmong and Red Dzao people who predominate here are no exception; Sapa's tourism explosion has engendered a new normal of interacting with outsiders, leaving minorities perhaps even more exposed.

I caught wind of what was happening in Sapa in late 2012. There was a buzz about girls who "go to China" or "get stolen" that if you were paying attention was impossible to miss. One only needed to chat with the minority women hawking textiles in the street, shoot pool with the proprietor of a hotel, or hang around Sapa O'Chau to begin to grasp the extent of the phenomenon.

It was hardly monolithic. Some girls were taken outright, but others went of their own volition, spurred by a bad home life, an abusive husband or some dreaded, inescapable fate. Phil Hoolihan, manager of the H'mong Sapa Hotel, told me how one of his staffers, a 16-year-old Black Hmong girl, tried to kill herself after her parents ordered her to marry someone she didn't love. She already had a boyfriend, but he couldn't afford the dowry — about $1,500, the price of a water buffalo — and the father said she had no choice. "So she ate the poison leaf," Hoolihan said, and he meant it literally. She was still in the hospital. "It was her escape method."

During the period in which Sapa O'Chau lost its three students, Gilbert had been running a tour guide class; the first two girls, the ones who set off together, were enrolled. One day they just stopped coming. "We still care about those kids a lot," he said. "But it just seems like almost a part of life here that it's not that shocking, not something people are still talking about today."

Those two never returned. But the third girl, Thi, actually made it back to Sapa. No one could say exactly how. But everyone knew she had resumed her job as a tour guide, the one she had held before she left town about a year earlier.

Gilbert said he knew Thi — knew her well, in fact. Thi had attended his class, but she dropped out because she couldn't deal with the rules or keep from fighting with the other kids. Gilbert hadn't talked to her about China, though. He hadn't talked to any of the ones who had returned about China. "I don't want to talk to them, really," he said. "I don't want to stress them out."

A student works at Sapa O'Chau, an organization that houses, teaches and inspires hill tribe children | (Samantha Falco/Courtesy Latterly)

I met someone who offered to introduce me to Thi, and she and I sat down one afternoon in the town square. (The names of some of the girls have been changed.) It was a cool, clear October day, free of the dense flash fog that can sweep in so suddenly and obscure this place. Thi, who was 17 when we first spoke in late 2012, wore traditional Black Hmong clothes, colored indigo with patches of intricate, psychedelic patterns. Her fine black hair hung in a long ponytail over the back of her handmade outfit. On the concrete expanse before us, women sat on tapestries laden with handicrafts and tried to flag down tourists, some of whom bit — the stuff was cheap — some of whom just observed, often surreptitiously through their camera lenses.

Thi's tale began one day at her room in town, when one of her girlfriends dropped by with a boy she'd just met. The boy, shy, hung around the door, then left for a few minutes and returned with another boy. The newcomer seemed nice enough, and after they departed Thi didn't think much of it. Later that day, though, she noticed her phone had been used to call an unrecognized number. When she dialed to see who it was, the second boy picked up. "Now we know each other," he said.

The next week, he called her, and they met again. He bought a shuttlecock, and they kicked it around the square with her friends. Then they went off on their own for a walk around the lake. When they settled on a bench, Thi texted with a girlfriend who teased her darkly. "Uh oh, first time, I don't know if you go to China or not," the friend said. Thi wrote back: "This time I go for sure!"

It was only a joke. But then the boy suggested they take a quick trip to Lào Cai, the lowland border town both an hour and a world away from Sapa. Just to walk around, check it out. Thi claims he slipped her a "medicine," a special drug that made her like him. The next thing she knew, she was on the back of his bike, headed down, down, down the mountain…

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