The U.S.-Mexican border is nearly 2,000 miles long. America's determination to keep illegal aliens out is matched only by their desperation to get in.
"The reality is that hundreds of thousands of people from all over the world are successfully sneaking into the United States," said Dave Stoddard, a 27-year Border Patrol veteran.
In spite of the massive resources invested in border security, hundreds of thousands, maybe millions, of undocumented aliens make it into the United States every year.
This is part one of a five-part series looking at how illegal immigration affects U.S. border security, the criminal, health care and education systems, as well as the economy. Watch the series this week on FOX News Channel.
Although some say illegal immigration seems to be out of control, others in government and private industry argue that low-wage, unskilled labor is critical to keeping prices down and America competitive.
"These people that are coming up here, including the undocumented, are good people that are enriching our lives. We do need them," said Juan Hernandez (search), a dual national and Texas resident who formerly represented Mexicans north of the border in the Mexican cabinet.
Rancher George Morin, who raises cattle along the Arizona border, has had several run-ins with illegal aliens near his property.
"I woke up real early in the morning, went over to the little dike right here behind the house, and there was about 600 people in the tank there," Morin said. "So I stood there and looked at them and got ahold of the Border Patrol and they actually loaded three Greyhound buses."
"The rest of the people were running off like quail," he added. "It was just insanity."
Morin added that not every run-in has been nonviolent. Two of his dogs were killed by illegal aliens; one had its head cracked open with a stick, and the other one was poisoned, he said.
In the 1970s, fewer than 100,000 workers entered the United States illegally each year. By 1990, that figure had doubled.
Since then, illegal immigration has exploded, with more than 1 million instances of foreigners being detained at the U.S. border last year.
Some Americans are even taking the law into their own hands, patrolling the borders they feel Washington has abandoned.
Experts say that possibly 12 million people live in the United States illegally — more people than live in Oregon, Oklahoma, Nebraska, Arkansas and Rhode Island combined.
The latest U.S. government estimate was that 7 million illegal immigrants lived in the country in Jan. 2000, more than five years ago.
What's most unsettling to many Americans is not the huge numbers of illegal aliens caught at the border, but the possibly millions more who are not caught.
"Can anybody explain to me why we shouldn't be paranoid about the southern border being porous?" asked Rep. Tom Tancredo (search), R-Colo.
Tancredo has obtained records showing that since the Sept. 11, 2001, terror attacks, agents from the U.S. Customs and Border Protection agency have stopped 132 nationals from countries considered a national security threat, including Syria, North Korea and Iran.
If those numbers are accurate, they may indicate that hundreds more from suspect nations made it across the border.
"The element that concerns me today is the terrorist element. Mainly, radical Muslims from the Middle East," Stoddard said.
Stoddard spent the last eight years in an area known locally as the "Arab Road," where ranchers recently found a prayer rug, a Koran and a diary written in Arabic.
Those who call for immediate action to better secure the country's borders are concerned about the millions who come to America to make a better life, but even more worried about the handful whose intentions are not so noble.
The intelligence reform bill (search) recently signed into law by President Bush calls for an increase in border staffing from 10,000 to 20,000 over the next five years. But the administration's fiscal 2006 budget calls for only 210 new agents next year.
"If we have another event like 9/11, or worse, and if that event is perpetrated by somebody who has come into this country illegally and if we have done no more to secure those borders than we have presently done," Tancredo said, "then the blood of everyone who's killed in that will be on our heads in the Congress and on the president of the United States."
Tomorrow's installment of this series will focus on how prisons in border states like Arizona are overflowing with with criminals who crossed the border from Mexico.
Click on the video box above for a complete report by FOX News' William LaJeunesse.