No one knows exactly how many people enter the United States illegally each year, but studies based on data from the Census bureau and other federal agencies indicate that there are now more than 10 million undocumented immigrants residing in the country. As many as three million of them may have entered the country just within the past year or so. Most of the illegal entry occurs on the US-Mexico border, with the main point being the border with Arizona.
The number of illegal immigrants crossing over from Mexico has grown steadily in recent years in spite of U.S. Border Patrol efforts.
Agents say for every illegal entrant they catch, there are probably several others who get through.
Although it may look like a wasteland to some, the desert is a delicate ecosystem that is being threatened by immigrant and drug-smuggling traffic.
One person who sees the impact is Kathy Billings, Superintendent of the Organ Pipe Cactus National Monument.
"There's many areas that are highly impacted by the trails and by the places where the people camp and just leave their trash and human waste,” she said. “There is a spider web of trails through the park that have been created by that illegal activity."
Intrusions from Mexico have also caused a rise in crime and vandalism on the Tohono O'Odham Indian reservation, which sits on the border. Reservation Police Chief Richard Saunders says his people are deeply offended by the trashing of land they hold sacred.
"As soon as you clean up an area, there are people coming back through it the next day, discarding trash and articles all over again. So, it is a constant battle there," he said.
In response to these local issues and concerns that terrorists could also slip across the border here, hundreds of civilian volunteers staged a watch along one 37-kilometer strip of the border in April.
They observed and reported illegal activity to the Border Patrol, but very few people tried to cross that section of the border while they were there.
The volunteers call themselves Minutemen after a famous rapid-response group in the American Revolutionary War. They were cheered by many local residents including Dylan Cron, whose property had been invaded by illegal border crossers.
"There are a lot of issues at stake right here and these guys are doing the right thing and my hat is off to all of them," he said.
While the Minutemen were watching the line, they were being watched by volunteers from the American Civil Liberties Union and Mexican federal police on their side of the fence. But there were no incidents of abuse or violence reported.
In the old mining town of Tombstone, newspaper publisher and Minuteman organizer Chris Simcox declared the movement a success.
"The spirit of the United States citizen having a can-do attitude and putting your boots on the ground and having the courage of your convictions is what this is all about," he said.
But not all US citizens on the border agree. Jennifer Allen, executive director of the Border Action Network, a pro-immigrant group, says the Minutemen intimidated Hispanics in the area by their very presence.
"Whether they are breaking laws or not, they are still furthering this hysteria and this anti-immigrant frenzy and this misguided notion of what is happening on the border and what needs to happen on the border," she explained.
People cross the border illegally in search of jobs that pay several times more than what they could earn at home in Mexico or other poor nations. But it falls to local governments, and not their U.S. employers, to pay for health care and other social services for the workers and their families.
Chris Simcox says his group wants the government to enforce laws against hiring illegal aliens.
"We want a grassroots effort nationwide. We want to set up pickets and protest every employer we can identify who is hiring illegals," he said.
But Jennifer Allen says that sort of talk ignores economic reality.
“We have a false economy in this country that is incredibly dependent on undocumented labor. There was just a call put out by the Yuma, Arizona Grower's Association to the Border Patrol to back off at some of their checkpoints in western Arizona because they could not get enough lettuce pickers to come in and harvest the crops," she said.
Arizona voters last year approved Proposition 200, which denies some social services to undocumented immigrants. Some immigration reform advocates favor a guest worker program, such as the one being proposed by President Bush, but critics see it as a backdoor amnesty for people who violated US immigration law and it faces stiff opposition in Congress.
Meanwhile, the immigrants keep coming.