Her account reminded me-painfully-of the legend of the Pied Piper of Hamelin. In the story, a piper shows up and asks for one thousand guilders for ridding the town of a plague of rats. Playing his pipe, he lures all the rats into the River Weser, where they drown. But Hamelin's mayor refuses to pay him. The piper goes back into the streets and again starts to play his music. This time "all the little boys and girls, with rosy cheeks and flaxen curls, and sparkling eyes and teeth like pearls" follow him out of town and into the hills. The piper leads the children to a mountainside, where a portal opens. The children follow him in, the cave closes and Hamelin's children-all but one, too lame to keep up-are never seen again.

Montserrat said that she was moved around a lot and often didn't know where she was. She recalled that she was in Detroit for two months before she realized that she was in "the city where cars are made," because the door to the apartment Alejandro kept her in was locked from the outside. She says she was forced to service at least two men a night, and sometimes more. She watched through the windows as neighborhood children played outside. Emotionally, she slowly dissolved. Later, Alejandro moved her to Portland, Oregon, where once a week he worked her out of a strip club. In all that time she had exactly one night off; Alejandro took her to see Scary Movie 2.

All the girls I spoke to said that their captors were both psychologically and physically abusive. Andrea told me that she and the other children she was held with were frequently beaten to keep them off-balance and obedient. Sometimes they were videotaped while being forced to have sex with adults or one another. Often, she said, she was asked to play roles: the therapist's patient or the obedient daughter. Her cell of sex traffickers offered three age ranges of sex partners-toddler to age four, five to twelve, and teens-as well as what she called a "damage group." "In the damage group they can hit you or do anything they wanted," she explained. "Though sex always hurts when you are little, so it's always violent, everything was much more painful once you were placed in the damage group.

"They'd get you hungry then to train you" to have oral sex, she said. "They'd put honey on a man. For the littlest kids, you had to learn not to gag. And they would push things in you so you would open up better. We learned responses. Like if they wanted us to be sultry or sexy or scared. Most of them wanted you scared. When I got older I'd teach the younger kids how to float away so things didn't hurt."

Kevin Bales of Free the Slaves says: "The physical path of a person being trafficked includes stages of degradation of a person's mental state. A victim gets deprived of food, gets hungry, a little dizzy and sleep-deprived. She begins to break down; she can't think for herself. Then take away her travel documents, and you've made her stateless. Then layer on physical violence, and she begins to follow orders. Then add a foreign culture and language, and she's trapped."

Then add one more layer: a sex-trafficking victim's belief that her family is being tracked as collateral for her body. All sex-trafficking operations, whether Mexican, Ukrainian, or Thai, are vast criminal underworlds with roots and branches that reach back to the countries, towns, and neighborhoods of their victims.

"There's a vast misunderstanding of what coercion is, of how little it takes to make someone a slave," Gary Haugen of International Justice Mission said. "The destruction of dignity and sense of self, these girls' sense of resignation…" He didn't finish the sentence.

In Tijuana in November, I met with Mamacita, a Mexican trafficking-victim-turned-madam, who used to oversee a stash house for sex slaves in San Diego. Mamacita (who goes by a nickname) was full of regret and worry. She left San Diego three years ago, but she says that the trafficking ring, run by three violent Mexican brothers, is still in operation. "The girls can't leave," Mamacita said. "They're always being watched. They lock them into apartments. The fear is unbelievable. They can't talk to anyone. They are always hungry, pale, always shaking and cold. But they never complain. If they do, they'll be beaten or killed."

In Vista, California, I followed a pickup truck driven by a San Diego sheriff's deputy named Rick Castro. We wound past a tidy suburban downtown, a supermall and the usual hometown franchises. We stopped alongside the San Luis Rey River, across the street from a Baptist church, a strawberry farm, and a municipal ballfield.

A neat subdivision and cycling path ran along the opposite bank. The San Luis Rey was mostly dry, filled now with an impenetrable jungle of fifteen-foot-high bamboolike reeds. As Castro and I started down a well-worn path into the thicket, he told me about the time he first heard about this place, in October 2001. A local health care worker had heard rumors about Mexican immigrants using the reeds for sex and came down to offer condoms and advice. She found more than four hundred men and fifty young women between twelve and fifteen dressed in tight clothing and high heels. There was a separate group of a dozen girls no more than eleven or twelve wearing white communion dresses. "The girls huddled in a circle for protection," Castro told me, "and had big eyes like terrified deer."

I followed Castro into the riverbed, and only fifty yards from the road we found a confounding warren of more than thirty roomlike caves carved into the reeds. It was a sunny morning, but the light in there was refracted, dreary, and basementlike. The ground in each was a squalid nest of mud, tamped leaves, condom wrappers, clumps of toilet paper and magazines. Soiled underwear was strewn here and there, plastic garbage bags jury-rigged through the reeds in lieu of walls. One of the caves' inhabitants had hung old CDs on the tips of branches, like Christmas ornaments. It looked vaguely like a recent massacre site. It was eight in the morning, but the girls could begin arriving any minute. Castro told me how it works: the girls are dropped off at the ballfield, then herded through a drainage sluice under the road into the riverbed. Vans shuttle the men from a 7-Eleven a mile away. The girls are forced to turn fifteen tricks in five hours in the mud. The johns pay fifteen dollars and get ten minutes. It is in nearly every respect a perfect extension of Calle Santo Tomas in Mexico City. Except that this is what some of those girls are training for.

If anything, the women I talked to said that the sex in the United States is even rougher than what the girls face on Calle Santo Tomas. Rosario, a woman I met in Mexico City, who had been trafficked to New York and held captive for a number of years, said: "In America we had 'special jobs.' Oral sex, anal sex, often with many men. Sex is now more adventurous, harder." She said that she believed younger foreign girls were in demand in the United States because of an increased appetite for more aggressive, dangerous sex. Traffickers need younger and younger girls, she suggested, simply because they are more pliable. In Eastern Europe, too, the typical age of sex-trafficking victims is plummeting; according to Matei of Reaching Out, while most girls used to be in their late teens and twenties, thirteen-year-olds are now far from unusual.

Immigration and Customs Enforcement agents at the Cyber Crimes Center in Fairfax, Virginia, are finding that when it comes to sex, what was once considered abnormal is now the norm. They are tracking a clear spike in the demand for harder-core pornography on the Internet. "We've become desensitized by the soft stuff; now we need a harder and harder hit," says ICE Special Agent Perry Woo. Cybernetworks like KaZaA and Morpheus / through which you can download and trade images and videos-have become the Mexican border of virtual sexual exploitation. I had heard of one Web site that supposedly offered sex slaves for purchase to individuals. The ICE agents hadn't heard of it. Special Agent Don Daufenbach, ICE's manager for undercover operations, brought it up on a screen. A hush came over the room as the agents leaned forward, clearly disturbed. "That sure looks like the real thing," Daufenbach said. There were streams of Web pages of thumbnail images of young women of every ethnicity in obvious distress, bound, gagged, contorted. The agents in the room pointed out probable injuries from torture. Cyberauctions for some of the women were in progress; one had exceeded $300,000. "With new Internet technology," Woo said, "pornography is becoming more pervasive. With Web cams we're seeing more live molestation of children." One of ICE's recent successes, Operation Hamlet, broke up a ring of adults who traded images and videos of themselves forcing sex on their own young children.