I followed Castro into the riverbed, and only 50 yards from the road we found a confounding warren of more than 30 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 CD's on the tips of branches, like Christmas ornaments. It looked vaguely like a recent massacre site. It was 8 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 15 tricks in five hours in the mud. The johns pay $15 and get 10 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 U.S. 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 U.S. 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 20's, 13-year-olds are now far from unusual.

Immigration and Customs Enforcement agents at the Cyber Crimes Center in Fairfax, Va., 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 I.C.E. 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 I.C.E. agents hadn't heard of it. Special Agent Don Daufenbach, I.C.E.'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 I.C.E.'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.

But the supply of cheap girls and young women to feed the global appetite appears to be limitless. And it's possible that the crimes committed against them in the U.S. cut deeper than elsewhere, precisely because so many of them are snared by the glittery promise of an America that turns out to be not their salvation but their place of destruction.

ENDGAME

Typically, a young trafficking victim in the U.S. lasts in the system for two to four years. After that, Bales says: ''She may be killed in the brothel. She may be dumped and deported. Probably least likely is that she will take part in the prosecution of the people that enslaved her.''

Who can expect a young woman trafficked into the U.S., trapped in a foreign culture, perhaps unable to speak English, physically and emotionally abused and perhaps drug-addicted, to ask for help from a police officer, who more likely than not will look at her as a criminal and an illegal alien? Even Andrea, who was born in the United States and spoke English, says she never thought of escaping, ''because what's out there? What's out there was scarier. We had customers who were police, so you were not going to go talk to a cop. We had this customer from Nevada who was a child psychologist, so you're not going to go talk to a social worker. So who are you going to talk to?''

And if the girls are lucky enough to escape, there's often nowhere for them to go. ''The families don't want them back,'' Sister Veronica, a nun who helps run a rescue mission for trafficked prostitutes in an old church in Mexico City, told me. ''They're shunned.''