It was August 1997, and I stood beside a manhole cover at Ninth Avenue and F Street in the border town of Douglas, Ariz., with a small gathering of police detectives, firefighters, and city workers. Cones diverted traffic around us. Frank Garcia, a hazardous-materials technician, knelt and ran a tube through one of the silver-dollar-sized holes in the manhole cover. He tested the air in the storm drain below for oxygen content and methane fumes.

"It’s clear," Garcia said. He pried back the heavy iron lid, revealing a black hole in the street. "You can go in."

I was here as a hydrologist to study the results of a flash flood. I would examine and measure sediment to determine the force and point of origin for the last flood that rushed through the catacombs under Douglas. None of the others wanted to join me.

The storm drains buried under Douglas, like those in other border towns, are an underground railroad for illegal immigrants entering the United States. A violent thunderstorm had recently struck at the same moment that 13 migrants were deep in the tunnel below us. The water had charged down street gutters and storm drains, filling the tunnel. The immigrants had only seconds to respond. These workers standing at the manhole, having already slogged through blocks of subterranean mud to recover bodies, had no wish to go back down into this hell.

I climbed down damp metal rungs into a narrow enclosure of cool, dank air. Twelve feet down, I stepped onto a bed of wet gravel and sand. I switched on my headlamp and looked north into a concrete tube three and a half feet tall. My light was swallowed by an infinite cone of darkness. To the north, the scene was exactly the same. I couldn’t help but imagine the howling sound of a flood approaching, the grip of sudden terror. I looked up at a bright circle of daylight. Faces peered down at me.

"I’ll be back soon," I said, and I crouched into the blackness.

The immigrants had gathered around midnight just across the border at the Hotel Yolanda in the small town of Agua Prieta, Mexico, on Aug. 6. They waited anxiously as a treacherous-looking smuggler, a coyote, looked them over. Every night they are here, in towns along the border, commuters heading north for work. They stop with their satchels and bags, waiting for an opportunity to cross, paying a coyote a few thousand dollars for safe passage. Immigration officials say at least 500,000 people enter the United States illegally every year (HCN, 5/15/06: The Immigrant's Trail).

They all face the same horrors — smugglers who threaten violence if they don’t pay, metal fences topped with razor wire, vigilantes with guns on the other side, and the U.S. Border Patrol, waiting with handcuffs and buses to haul them back to Mexico. Whenever border protection has been tightened, death rates have increased. When a 14-mile-long wall was built along the Tijuana border with California in 1994 and patrol officers stepped up their efforts, immigrants took more risky crossings, both across the desert and underneath it through underground drainage tunnels. The local death toll leapt 600 percent.

On this night, most of the migrants at the Hotel Yolanda were from hundreds of miles to the south. There was a 24-year-old woman from Mexico City, a man from Guanajuato on his way to see family in Chicago, an 18-year-old from Michoacán. There were two southern Mexican women — one with her 2-year-old daughter balanced on her hip. They numbered 13. The coyote told them to come with him.

Lightning flashed in the north, illuminating the dark folds of a thunderstorm that rose 50,000 feet into a black sky. As the migrants scrambled into a ditch, ducking under a sweeping spotlight, lightning was the least of their worries. The coyote waved his hand, sending young men running into the open, decoys for the Border Patrol. In that instant the 13 slipped through a gap cut in the metal border fence and moved swiftly to a concrete hole in the side of a ditch on the U.S. side.

The coyote quickly explained that he would leave them here to enter the tunnel. They were to travel 10 blocks north, marking blocks by the manholes above. At the tenth manhole, they would climb up, open the cover, and meet the next contact, who would show them to a nearby safehouse. Carrying a few flashlights, the travelers did as they were told, crawling into a concrete shaft under F Street.

They had gone nine blocks when they heard a sound up ahead like wind howling down a glass tube. They stopped, casting their flashlights ahead, seeing nothing but the infinity of this circular passageway. The sound grew into a roar.

I moved uncomfortably along the same constraining tube, my shoulders pulled in, head ducked toward my knees, hands pawing at wet gravel. I looked back at a bright crescent in the distance, a shaft of daylight beaming down the hole I’d climbed down. It looked as if it was floating in space. Claustrophobia was so close I could feel it feeding on my fears.

I took note of anything I found — the plastic wrapper from a burrito, pieces of metal, rusted car parts, willow and poplar leaves, scales of junipers. I also found some shredded bits of soaptree yuccas and creosote bush from the desert where this flood had started. I recorded the size and geologic type of pebbles and sand grains. I tried to keep my mind on findings, data, but I kept looking back over my shoulder and then peering ahead. I had to steady my breath in this underground tightness.

People died in here, I thought. I knew how it happened. Witnesses had told me stories. The night of the flood, a woman who lives along F Street had woken to the sound of what she thought was pounding and screaming. She walked outside.The rain had just let up. Gutters were clogged. She saw water bubbling out of the holes in a manhole cover. Someone had tried to escape there, but failed. Others never made it to manholes. They were tumbled end over end and died with their lungs full of mud.

After a few minutes I looked back and saw only a pinprick of light, a faint star where I had begun. I could feel the weight of streets and grocery stores and houses pressing down on me. We make them commute in this underworld, I thought, while we spend our days up in the light, moving freely, eating cheap produce, wearing maquiladora clothing. Workers scamper under our feet unseen like ants (estimates of illegals in the United States range from 7 million to 20 million) and only now was I aware of them, drawn by a tragedy into their secret corridor. I placed my hands against the concrete curve surrounding me. This was not a place for humans. It was built for water alone.

As I crawled forward, I came upon a wooden-handled sledgehammer. Half-buried in wet sand, it was difficult to unearth. The tool was down here for a reason. People left sledgehammers and crowbars, tools for prying or pounding open manhole covers — keys to doorways that open into the land of the free above. I set the sledgehammer back into the sand, knowing it would come in handy for someone else.

At about 1:30 on the morning of the flood, city workers arrived at the Douglas port of entry on Pan American Avenue to remove debris that was causing the drainage ditch below the road to overflow and flood businesses in a nearby shopping center. Frank Amarillas, the port director, was watching a backhoe excavate the debris when he spied a bundle in the muck. Thinking it might be a bag of marijuana washed out of the tunnel, he reached down to find that, instead, it was a woman’s body.

A search quickly turned up children’s clothing and toys, and then a duffel holding personal belongings and papers describing a woman in her mid-30s from Colima, Mexico. At 6 a.m., searchers found a second body, and after sunrise, a third. In the afternoon, both sides of the border crowded with onlookers, sweating out a 100-plus-degree day to watch the recovery effort. A young boy sat throwing rocks into the mud-filled ditch, distractedly hitting a wad of fabric and debris. A man walked over and quickly asked the boy to stop. He called searchers over. The fabric turned out to be a piece of clothing. Under it were three more bodies.

Miraculously, only eight of the 13 travelers had perished. The little girl and her mother and another woman managed to climb the ladder at Ninth Avenue. With their backs they pushed open the manhole cover and escaped into the night. Downstream, two men reached another manhole but a car was parked on the lid. Trapped, they panicked as floodwater rose up the 12-foot shaft and swirled around their collarbones. They breathed with their mouths against the iron cover. Then the water receded, leaving them alive.

Border Patrol agents quickly found the survivors, including the two women and the child, and detained them. As soon as reports were filed, they were delivered straight back to Mexico, where they most likely began looking for another way across. Meanwhile, the last two bodies had not been located.

Searchers combed the tunnels under Douglas, pushing through deep mud, but there was too much debris to sort through in too small an enclosure. Relatives of one of the missing men, both in the United States and in Mexico, made desperate phone calls to officials, but heard nothing. They came to Douglas from both sides of the border and demanded the search continue. But the search was called off. The bodies simply could not be found.

Twelve days after the search ended, the tunnels gave up the missing men. A second flood washed their two bodies free, along with most of the remaining debris and mud.

Not long after that, I arrived, researching merely one of countless floods. My findings would answer scientific questions and nothing more, showing the flood began in the Perilla Mountains northeast of town, and that it carried more water and debris than Douglas storm drains could handle.

After I gathered my numbers and notes, I inched my way around to face toward the light a few blocks away. When I looked for it, though, I saw nothing. I turned off my headlamp, and still there was only a black void before me. I felt the burden of the entire desert pushing against me, countless barren waterways leading to town and into this meager pipe in the ground. I could also feel the press of humanity on my back, drug smugglers and fruit pickers pushing into the United States. I looked both ways, all directions. I was suspended in absolute darkness. There was no north and south in here, no up and down.

I scrambled ahead, moving quickly, scraping my hands, the top of my head. My knees hit my chin as the pinpoint light came into view. I moved awkwardly until I reached the ladder, breathing hard, peering up at a blinding circle of daylight. I climbed metal rungs and emerged into a hot blue sky, relieved to see the traffic going by, the storefronts, the land of the free.

Craig Childs is author of several books including The Secret Knowledge of Water and The Desert Cries. His newest book, House of Rain: Tracking a Vanished Civilization Across the American Southwest, will be out this winter.