About 1 mile deep into the drafty tunnel under this hilly frontier city, a flashlight beam cuts through the pitch-black darkness and illuminates a yellow line painted on the concrete wall: the U.S.-Mexico border.
Just beyond the boundary a graffiti-message believed to have been scrawled by U.S. law enforcement warns intruders: "USA Tunnel Rats. Este lugar es de nosotros" - This place is ours.
Inside the largest known tunnels on the border - two passages that make up an enormous drainage system linking Nogales, Mexico, with Nogales, Ariz. - migrants stumble blindly through toxic puddles and duck low-flying bats. Methamphetamine-addicted assailants lurk. Young men working as drug mules lug burlap sacks filled with contraband.
There are shootouts and rapes. Rising floodwaters sweep people to their deaths. U.S. Border Patrol agents pursue smugglers in frenzied chases, insults and threats echoing as they go. And tangles of rebar metal - points sharpened by smugglers - gouge people who get too close to some walls.
"It's another world down there," said Pat Thompson, a police detective in Nogales, Ariz. "You don't know what to expect."
As the United States prepares to fence much of the border above ground, the situation below ground could grow increasingly chaotic. Authorities have discovered dozens of illegal tunnels in recent years, including a nearly half-mile passage connecting Tijuana with San Diego.
Illegal immigrants have breached drainage systems all the way along the border, from El Paso, Texas, to San Diego. Most of them are of the claustrophobic, crawl-through variety that prevents large-scale incursions.
The Nogales tunnels, by comparison, are superhighways.
Once open waterways, today they stretch for miles under the traffic-clogged streets of both cities, bending and zigzagging roughly parallel to each other.
In the smaller one, called the Morley Tunnel, an ankle-high stream of raw sewage and chemical runoff from factories in Mexico flows. The neighboring Grand Tunnel is as much as 15 feet high and wide enough to fit a Humvee. Dozens of illegal immigrants can travel through it at one time.
Above ground, fences, sensors and stadium lighting clearly separate the two cities. Underground, they remain linked of necessity by the system built decades ago to channel torrential rains.
The tunnels doubled as smuggling routes from the beginning. For many years, gangs of children took control of the passages. Nogales police once encountered Mexican soldiers on the U.S. side, prompting a brief but tense standoff.
In recent years, the U.S. Border Patrol has had some success stemming the underground flow of illegal immigrants and drugs by installing heavy steel doors, surveillance cameras and sensors. But when heavy rains this summer triggered floodwaters that tore down the gates, smugglers ripped down the cameras and shattered the lights and siren used to discourage incursions - and the chaotic human flow resumed.
From July through October, agents apprehended 1,704 illegal immigrants in the tunnels, a nearly five-fold increase from the previous six months. Agents seized more than a ton of marijuana from tunnel arrests during the same period. In July, bandits raped two women from Oaxaca, Mexico, in the tunnels on the Mexican side.
This summer, five people are believed to have drowned after being caught in floodwater. Two others fell into a sewage drain branching off one tunnel and were carried 9 miles before being discovered alive in a shaft near a sewage treatment plant.
Imelda Guevara Lopez, 17, said she survived by never letting go of her friend's hand as she struggled to keep her head above the flow of raw sewage. Lopez, whose backside was shredded by the concrete walls, told workers at a migrant shelter in Mexico that she would never again enter the underground.
"I prefer working in the fields and being poor but alive," said Lopez, who went home to Hidalgo, according to an account in a Mexican newspaper.
Patrolling the tunnels is a tactical nightmare for law enforcement on both sides of the border, mainly U.S. Border Patrol agents and Grupo Beta, Mexico's migrant safety force.
U.S. agents often can't go into the Morley Tunnel because overpowering ammonia and chlorine smells leave them nauseated and dizzy. On the Mexican side, some stretches of the tunnel are so low Grupo Beta agents ride all-terrain vehicles lying on their stomachs.
Teams of U.S. agents enter the Grand Tunnel daily, sometimes toting M-4 assault rifles. But their high-tech night vision goggles are rendered almost useless in the tunnel's black-hole-like reaches.
"It's so dark, you feel vertigo - like the walls are coming in on you," Agent Scott Wencel said.
A distant flicker of flashlights - sometimes half a mile away - usually signals an approaching group. They could be drug traffickers or bandits or illegal immigrants. Some have walked 1 mile after descending from Avenida Reforma in Nogales, Mexico, taking advantage of the cracked grate in front of Elvira's Bar.
"They climb down every day ... people from all over Mexico," said 62-year-old Sebastian Flores, an auxiliary traffic police officer in Nogales, Mexico.
The groups cross the yellow line in complete silence - the only sounds the distant hum of traffic, the chirping of crickets, the scurrying of rats. Sometimes the tunnel itself seems to be alive as the humming and air flows generate a pulsing, low groan.
The darkness is so thick that migrants sometimes cross within an arm's length of U.S. agents without noticing. That's the agents' preferred tactic: lying in wait, pressed against the walls, letting groups pass before pouncing and cutting off any escape back to Mexico.
Some illegal immigrants are so startled that they run smack into the walls, agents say. During one sweep last December, when smugglers heard them coming, agents yelled out: "Somos migra!" - Border Patrol. They ordered the group to stop.
"Migra go home!" came the shouted reply as the people ran back into Mexico.
If the migrants manage to evade agents in the tunnels, another huge challenge remains: getting out. People pop up from manholes into the middle of busy streets. Curb storm drains are often too small, so smugglers use 20-ton hydraulic jacks to pry them open so people can squeeze through.
Some grates have been opened so often that Nogales city workers have placed huge boulders and concrete blocks on top of them.
Now many migrants walk 1 mile past where the border is marked underground to reach the open end of the drainage tunnels. Outside again, they climb an embankment to waiting cars.