By Laura Smith-Spark
From 1 April, groups of volunteers from across the US will spend up to a month camped out in the inhospitable Arizona desert.
Some 580,000 illegal immigrants were detained in Arizona last year
But this is no ordinary hunting or camping trip.
Armed with night vision goggles, radios and light aircraft, their quarry are the hundreds of immigrants who each night seek illegally to cross the wire fence separating the US from Mexico.
While the volunteers insist they will simply "observe" the new arrivals' movements and alert the US Border Patrol, human rights groups fear the eruption of vigilante violence.
Called the Minuteman Project, the operation has already strained international relations between Washington and its southern neighbour.
Washington accuses Mexico of not doing enough to stem the flow of illegal immigrants from across Latin America into the US, while Mexican officials condemn vigilante attacks near the border.
President George W Bush has so far delayed implementing plans announced in 2004 for a guest-worker scheme which would legalise immigrants' presence.
And for those seeking to join up to six million undocumented Mexican workers already in the US, the crossing is about to get tougher.
Founder James Gilchrist describes the Minuteman Project - named after militiamen of the American Revolution - as vital to protect the country from the security threat of uncontrolled immigration.
It is "a grassroots effort to bring Americans to the defence of their homeland", he says, which is "devoured and plundered by the menace of tens of millions of invading illegal aliens".
His website claims more than 1,000 people of all ages and backgrounds have registered to patrol through April.
But the project's statement that it has "no affiliation with, nor wants any assistance or interference from any separatists, racists or supremacy groups or individuals" does little to reassure human rights groups.
Jennifer Allen, of Border Action Network, based in Tucson, Arizona, tells the BBC News website her campaign group is seeking to force the authorities to take action to prevent any potential vigilante violence.
"This is really an issue of outsiders coming in, creating a situation about immigrants, fanning the flames of racism and riling up people's xenophobia," she says.
The US Border Patrol made over 500,000 arrests last year
"It's bringing people here from across the country, whereas the local people who deal with the fact they live on the border and deal with immigration routes are not the ones doing this."
Mark Potok, of the Southern Poverty Law Centre in Alabama, recognises the Minuteman volunteers' stated intention to patrol peacefully.
But he is concerned the organisers will not be able to screen all participants for links to white supremacist groups.
"We think they are pouring gas on the fire," he says. "We think it's very possible we will have some kind of eruption down there."
But US Border Control spokesman Jeff Benadum insists groups such as the Minutemen volunteers have a "constitutional right to assemble peacefully", although they cannot legally detain anyone.
"We have no reason not to feel everything they are going to do is law-abiding," he says. "As long as it doesn't interfere with our course of action, it's their due right to be out there."
Border Patrol officers are kept busy in the Arizona desert. Last year they made 580,000 arrests, up by more than a third on the year before and more than half the national total.
More people cross in Arizona since security was raised in California
Once apprehended, over 90% of illegal immigrants are voluntarily taken back over the Mexican border. Many will turn straight round and try again.
They are the lucky ones. The less fortunate die trying to cross the desert wilderness - some 223 of them last year, says volunteer group Humane Borders, most killed by thirst.
The growth in migration across the 200-mile-long stretch of Arizona border, despite the risks, is partly due to increased security in California since 1994.
Humane Borders attempts to prevent people dying on the five-day trek through Arizona's baking heat by putting water stations in the desert.
Volunteers try to save lives by setting up water stations in the desert
But, says Jeff Passel of the Pew Hispanic Centre, no amount of danger will deter immigrants as long as a better life beckons beyond the wire - dooming current border policies to failure.
"The one thing that seems to be behind the migration is jobs - and the US government has done very little to cut off employment," he says.
"No matter how hard you make it, it's still better than the prospects they have back home."