BISBEE, Ariz. -- Three thousand, five hundred feet above Arizona's Huachuca Mountains, a man peers down from a single-engine Cessna TU206 and spots a spider web-like network of trails.
"For us to see a trail up here, it's well worn. That means it's active," says the man, a Vietnam veteran in his 60s who created BorderInvasionPics.com, a website that publishes photographs of illegal immigrants crossing the U.S.-Mexico border.
Frustrated with federal efforts to combat illegal immigration, as well as human and drug trafficking, he is one of a growing number of independent border-watchers bent on exposing what they say is a worsening climate along the 372-mile Arizona-Mexico border.
He declined to be identified due to threats he's received since launching his website in late 2008. Someone, he says, even placed a map in his mailbox that marked his residence with an "X."
"In other words, we know where you are," he said. "I'm always alert and I'm always armed. There's nothing else I can do."
In the plane's cockpit is Glenn Spencer, president and founder of American Border Patrol. Headquartered in Hereford, an unincorporated community just north of the border, it bills itself as the only non-governmental organization that monitors America's southwest border on a regular basis, generally by air.
Spencer, 73, has been acting as a watchdog of U.S. Customs and Border Protection since 2003.
"I'm trying to develop a way to evaluate how well the Border Patrol is doing," he said. "I've been at this a long time. I know the problem with the border. It's an old axiom: If you can't measure it, you can't improve it."
Arizona's heated immigration debate, fueled in large part by a new law that allows authorities to detain anyone suspected of being in the country illegally, has stoked the fire driving both men to expose what they say is an "open" border. Spencer said he spends at least three or four hours a week monitoring the border from the sky.
He funds his operation with help from private donations and direct mail solicitations, and his primary wish is to determine exactly how many people enter the U.S. illegally every year.
The illegal immigrant population in the U.S. was estimated by the Center for Immigration Studies to be about 11 million in 2008, down from roughly 12.5 million in 2007. According to U.S. Customs and Border Protection, 556,041 border-crossers were apprehended in fiscal year 2009, down from 723,825 in 2008.
"The first thing you do in any system analysis of any project is to determine the score," Spencer said. "The job of the Department of Homeland Security is to prevent all unlawful entries. That is an impossible goal, but what should be done is to set a figure of merit -- a goal of, let's say, 20,000 people getting past Border Patrol and into our country. What we need to do is set a goal. We've got to measure this."
That goal can be met, Spencer said, by using "modified geophysical seismic cables" like the ones used during oil exploration. Those cables could be buried in a small trench along the border, perhaps at two different locations to determine direction of travel.
"I believe a good statistician could design this thing," he said. "It's doable."
While in the jumpseat of Spencer's Cessna, the creator of BorderInvasionPics.com peers down into the sheer cliffs and canyons and spots several Border Patrol vans in the crevasses.
He says those agents are stationed at known smuggling corridors just north of border cities like Nogales and Sesabe, and they probably can't detect experienced human and drug traffickers because of the high variance of elevation. In May and June alone, his website posted videos of roughly 200 "border intruders" crossing Arizona cattle ranches, a volume of traffic he says is unknown to most Americans.
"Living here, you're aware of the problem," he said. "But until you actually get out and hike around, you don't realize the extent of the problem. And the more I got involved, and when I realized how big it was, I realized the rest of the country couldn't really understand it unless they saw for themselves. As they say, a picture is worth a thousand words."
Using battery-operated digital cameras with infrared sensors designed to photograph wild animals, he says he spends roughly 40 hours a week tracking illegal immigrants. Once the $200 cameras are activated, they can capture still photos or video.
Those cameras have been placed on public land and -- with the owners' permission -- private land along 80 miles of the border, from as far east as Douglas to west of Nogales.
"I try to spread out so I can get a general idea of overall activity," said the website creator, who relies on private donations to fund his operation.
Jim Gilchrist, who founded the Minuteman Project in 2004 as an advocacy group to monitor the flow of illegal immigration, said the men represent the "second wave" of civilian border enforcement.
"I welcome these groups that are organizing," Gilchrist told FoxNews.com. "They're independent. Certainly it's a second wave of civilian border patrol enforcement."
But not everyone is a fan. Mark Qualia, a spokesman for U.S. Customs and Border Protection, said in a statement to FoxNews.com that the agency does not endorse private groups taking matters into their own hands, warning against potentially "disastrous personal and public safety" consequences.
"CBP appreciates the efforts of concerned citizens and non-governmental organizations as they act as our eyes and ears," the statement read. "CBP strongly encourages concerned citizens to call the U.S. Border Patrol and/or local law authorities if they witness or suspect illegal activity."