By David Willis
BBC News, Nogales, Arizona
As the United States prepares to fence much of its border, the new
battleground for agents chasing illegal immigrants is not above ground
but beneath it.
There are not enough agents to patrol the tunnels constantly
In Arizona officials have found that hundreds of people are tunnelling their way into the country from Mexico.
Some tunnels cannot be sealed because they are vital
drainage links, but it is people, not water, that are flowing through
I followed border patrol agent Gus Soto into another world.
One that is dark and dangerous, the new superhighway not only for illegal immigration but also the trafficking of drugs.
The tunnels which run beneath the border town of Nogales were eerily silent apart from the scurrying of rats.
Gus and his men were on the lookout for people
traffickers, highly-organised gangs for whom a human cargo is worth
more than a cargo of narcotics.
As we made our way walking through a labyrinthine
network of passageways, Gus told me that the traffickers, or "coyotes",
regularly charge up to $3,000 (£1,500) per person.
With 10 or more people per journey, that amounts to a highly profitable business.
Venturing further into this subterranean world we found clothes strewn around.
Gus told me that before making it to the surface the
coyotes often give their clients a fresh wardrobe to help them blend in
with their new surroundings.
And not far from the clothes were canvas bags, now discarded but once packed with drugs.
Gus told me the marijuana had probably been passed up to
gang members waiting on the street above through one of the scores of
exit points emanating from the main tunnels.
Since California and Texas plugged gaps in their borders
in the late 1990s this part of Arizona has been the favoured route for
The area around Nogales accounts not only for almost
half the arrests, but also half the drug seizures made by US border
Immigrants climb out of the tunnels through manhole covers
A mile from the entrance the tunnel narrowed as we neared the border.
A crudely-drawn yellow line across the ceiling and down
the wall was the only indication of the division between Mexico and the
We popped up through a manhole cover a few feet from the official border post.
This is how the coyotes get their charges into the
country, waiting until the coast is clear and then shooing their
charges through storm drains or manhole covers to the surface.
We had only just left the main tunnel when Gus got called to a second.
A group of migrants had been spotted lurking in the inky blackness.
But as he and his men moved in with their guns drawn the group shrank back to the Mexican side, tantalisingly out of reach.
We met Sonia at her home in Arizona.
She was nine months pregnant when she came through the tunnel and now lives in constant fear of being deported.
She told me she was lucky to have made it through the tunnel alive.
"It was pitch black and the water was up to our knees.
The smugglers made us hold hands so we wouldn't fall. It smelt of dead
animals and dead people," she said.
Sonia told me that people will continue using the tunnels because the poverty in Mexico is so bad.
"They'll keep coming because there is no choice," she said sadly.
Poverty, it seems, breeds enterprise.
Border agents recently discovered another new tunnel and plan to beef up patrols as a result.
But such is the challenge above ground they have not got the staff to man the tunnels 24 hours a day.
Theirs is a cat and mouse game with the people traffickers. As you close one gap it seems another one opens up.
Such is the allure of a new life in this land of immigrants.