On February 23rd, a North Korean girl just shy of her seventeenth birthday walked across the frozen Tumen River into China, becoming at once a runaway and a refugee. Song-hee (as she asked to be called) is a small, moonfaced girl with a faint constellation of acne trailing across her broad forehead. She came from Musan, an iron-mining city, where she had been in her junior year of high school. Her favorite subject was math. After graduation, she intended to go to a teachers’ college in the nearby city of Hoeryong. “I’m not the best in my class, but I have passion,’’ she told me. Although education in North Korea is nominally free, students buy their own lunches and books and are expected to provide monetary gifts for their teachers, who are so poorly paid that they cannot survive without the extra income. Song-hee’s parents had been saving money for her, since neither of her two brothers showed academic promise. “If anybody goes to college in our family, it should be you,’’ Song-hee’s father had told her.

Then, on November 30, 2009, the North Korean government announced that it was devaluing its currency. Henceforth, all existing North Korean money would be worthless, and small allotments of new money would be given to each family. The life savings of members of the nascent middle class were reduced to a handful of paper, worth about fifteen dollars on the market.

When a classmate of Song-hee’s confided, a few months later, that she planned to escape from North Korea in just two days, Song-hee impulsively accepted an invitation to go with her. Song-hee’s home was less than an hour’s walk from the Tumen, where her friends used to swim. Song-hee, who is afraid of the water, sat on the banks of the river, watching and fantasizing. Like the others, she wanted to see the world.

“We all knew there was no hope for anything to get better in North Korea,’’ she told me. “Sometimes we’d say, ‘Hey, if we crossed the river we’d be in China, but there are too many soldiers.’ ’’ Song-hee also knew that, if she crossed the border, she could be picked up by the Chinese police and sent back to face sentencing in a labor camp. The customary term is anywhere from six months to three years. But her friend had a relative living in China, and contacts who knew the best places to cross. “I decided if I did not take this opportunity I might not have another,’’ Song-hee said.

Song-hee left home early one morning without telling her parents. It was impossible to cross in Musan, so the girls walked past the main athletic stadium and into the mountains north of the city. After hiking three hours through the mountains, they descended to a bend about three miles from downtown Musan—a spot where the river is no more than a hundred feet across. Although it was broad daylight when the girls got there, the border guards couldn’t see this section of the river from their concrete pillboxes. They walked across the ice as quickly as they could and clambered up the embankment into China. From there, they made their way to Yanji, a city fifteen miles from the border, in the part of China once known as Manchuria.

Yanji is a flat, drab city with little to distinguish it from any other third-tier city in China, except for the mélange of signage in Chinese and Korean, which are both official languages. Of the city’s population of four hundred thousand, about one-third is ethnic Korean.

China’s economic boom has largely bypassed this part of the country. North Korea is an insurmountable barrier to what would otherwise be lucrative trading routes into South Korea, Japan, and Russia.

I have been travelling regularly to Yanji since 2003. When I went in March, the city was filled with abandoned construction projects, their steel girders stabbing the sky from concrete shells. From my room in the Yanbian International Hotel, I stared across a parking lot into one of these buildings, which was in the same state of incompletion as when I had last visited, a year earlier.

The city is among the coldest in China, and dog-meat soup is its culinary specialty. Many journalists, diplomats, and aid workers come to Yanji because it is often the first stopover for people escaping North Korea. The large ethnic-Korean population makes it relatively easy for them to blend in, and, after a century of migration across the border, some have relatives in China.

In the nineteen-nineties, the first defectors from North Korea were strong young boys who, though on the verge of starvation, could swim across the river. These days, the majority are women. It is easier for them to slip away from their day jobs, and also to get situated. The Chinese countryside is always short of women; China’s one-child policy, among other factors, has led to a skewed birth rate, and women tend to migrate to the cities. North Korean women are in demand in the sex industry—the latest big thing is live Internet feeds of women stripping for South Korean men—and many more have married Chinese men, although the marriages aren’t recognized by Chinese law. More recently, middle-class women in their fifties have been crossing the border to work as maids, babysitters, and cooks.

Whatever these women do in China, they do discreetly. Since the 2008 Olympics, China has installed concertina-wire fences and security cameras along the river near major cities. The police sometimes stop vehicles at random near the border to inspect them for undocumented North Koreans, and border residents can collect rewards of between a hundred and five hundred Chinese yuan (from fifteen to seventy-five dollars) for reporting defectors.

I met Song-hee twenty days after her arrival in China, through an activist who works with North Korean escapees. Fearing arrest by the Chinese police, she had barely left the house where she was staying with her classmate’s relative, and was reluctant to meet me. My contact had borrowed an unoccupied apartment from an ethnic Korean in a neighborhood of identical nineteen-nineties mid-rises. To insure that Song-hee wasn’t spotted with a foreigner, we arrived separately. I took the battery out of my mobile phone in order to avoid being tracked.

Song-hee sat cross-legged on the living-room floor, which was covered, in typical Korean style, with yellow vinyl. She wore a red spangled turtleneck underneath a black parka with a furry collar. She didn’t remove the parka as we spoke, even though her bangs were plastered against her forehead with sweat. She had never met a foreigner, and her nervousness grew when the interpreter told her that I was American. “Oh, evil Americans are our enemies,” she said. When she calmed down, the words came tumbling out, breathy and girlish.

Song-hee told me that she had lived better than most North Koreans, and, with her flushed complexion and plump fingers, she didn’t have the telltale signs of malnutrition manifested by many other North Koreans I had met in China. She had little memory of the famine of the nineteen-nineties, in which two million people—ten per cent of the population—died of starvation.