To those who traffic in humans, the slaves are nothing but cargo.
Undocumented workers, lured to the United States with the promise of a better life, often end up being sold into slavery.
The traffickers take extraordinary measures to get the workers inside the United States.
Some come hidden in shipping containers, stashed under the flatbeds of pickup trucks, or packed like sardines in the trunks of cars. Others are even rolled up in carpets.
Smugglers and those who traffic slaves prey on the desperate.
"It's almost always based upon a vulnerability. Poverty, despair, poor education, little prospects," said Wan Kim, the assistant attorney general in charge of the Justice Department's Civil Rights Division.
"Again, everyone trying to reach for the brass ring -- and that is America for many people across the world," Kim continued. "And so when they get a hope of an opportunity to do something significant, to make a life for themselves, in many cases to help bring money back to the family that they're leaving behind -- they jump on it, and they're lead by deceit to come to America and then all the promises are broken."
The Global Trafficking Trade in the U.S.
In an effort to crack down on smugglers, the Justice Department has set up 42 task forces nationwide to work with state and local officials.
From 2001 to 2006 the department prosecuted 360 people for human trafficking and modern day slavery -- a 300 percent increase over the previous six years.
To date the government has helped more than 1,000 victims from 72 countries escape the multibillion dollar smuggling industry that fuels modern day slavery.
The numbers are staggering.
Government estimates say 800,000 people are bought, sold and smuggled worldwide annually. As many as 18,000 of them make their way to the United States from all corners of the world from at least 72 countries, including Cameroon, Egypt, Indonesia and Korea.
Dozens of smuggling rings identified by the government traffic people in the United States, part of what is now a multibillion dollar black market industry.
'Lucrative Underground Activity'
Kim noted, "This may be the second or third most lucrative underground activity in the world."
"If you think about the typical underground illegal activity -- selling guns, selling illegal drugs -- once you sell the contraband, it's gone. You get the money and then you go find some more contraband to sell. Selling a human being is something that can be done over and over and over again. A woman forced into prostitution can generate thousands of dollars per night."
In 2005, the government convicted members of the Carreto smuggling cartel of promising scores of poor women from Mexico marriage and a chance for a better life in America, only to force them into sexual slavery in Queens and Brooklyn, N.Y.
The women -- some forced to work in a filthy brothel -- were told to charge johns $25 to $35, of which they didn't see a penny.
They were also beaten to keep them under control, and their movements were restricted.
"This kind of modern day slavery is hard to imagine in the United States. I think these cases are so painful to see, and it's often hard even for the victims to recognize that they are victims," said Julie Myers, assistant Homeland Security secretary in charge of Immigration and Customs Enforcement or ICE.
"It's disgusting," Myers said.
"These organizations are run by some of the very worst criminals in our society. They treat human beings as cargo, as commodities to prey upon for a simple profit."
Working From Sunup to Sundown
Some victims become slaves because they are too poor to pay their smuggling transport fee.
Seven years ago Jose Martinez, formerly of Mexico, was toiling in a Florida slave camp's tomato fields from dusk to dawn.
He told ABC News a smuggler sold him for $350.
Each night Martinez's captors locked him in a trailer with 28 other workers. He was under constant watch, and under threat of violence.
He finally ran away after four months of captivity.
In a similar Florida slavery case, Ramiro Ramos and his brother Juan were sentenced to 15 years in prison in 2004 for enslaving farm workers who worked from sunup until sundown.
Hundreds of undocumented workers have been warned they would be shot if they tried to leave.
"They had been told that if you try and leave, you'll be fed to the alligators. They heard that people had been beaten," said Laura Germino, coordinator of the Anti-Slavery Campaign for Immokalee Workers, a nonprofit worker support group.
"You are supposed to be able to go to a place with wages and good working conditions. In this situation, people had no freedom of movement due to the extreme atmosphere of coercion, of violence, of fear that they were going to be killed," she said.
The traffickers use not only violence to control slaves, but also the threat of deportation and arrest. Undocumented workers are often told they will be sent to prison and face even worse conditions.
Melanie Orhant, managing attorney of Break the Chain, a resource group for enslaved persons, said "the psychological coercion is a very large factor."
"Physical coercion, rapes, beatings, sexual assault, locking people in closets, definitely happens," Orhant said, "but the primary tool, one of the first tools that they use is psychological coercion, isolating people, scaring people and putting major fear in people."
So the victims stay hidden.
Signs of Slavery
"I think that one of the things that is very, very important to convey to Americans is that this is a problem that exists in local communities and it's a problem of recognition and awareness," said the Justice Department's Kim.
"It could be happening in your neighborhood and if the human traffickers are doing a good job, no one knows about it because it's an underground crime. If they're doing their jobs well, the victims are so scared, so subjugated, so victimized that they will not reach out for help," he added.
Authorities and support groups say to watch out for signs that could indicate a person is being enslaved: children who do not go to school, adults who appear to be servants, working in and around homes, but who never leave or talk with neighbors.
Officials are asking the public to be more aware.
Terrified victims often need a helping hand to free them from suffering in the shadows.
Pierre Thomas' series "Slavery in America: Living in the Shadows" continues Tuesday on "World News With Charles Gibson" and ABCNEWS.com.
You can also learn more from the Department of Health and Human Services' Rescue & Restore Victims of Human Trafficking public awareness campaign.
If you have information about human trafficking in the United States, please contact the the Department of Health and Human Services' National Human Trafficking Resource Center or call this toll free number: 1-888-3737-888.