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NOGALES, Mexico - The cocaine travels north through the sewer. Sometimes the traffickers send it floating in bags on a river of wastewater. Sometimes they crawl with it through mud and human excrement until they hit U.S. soil. As the U.S. border wall rose just north of this city, the drug trade here has been driven underground. Mexican and U.S. patrols have found tunnel after tunnel drilled into an 80-year-old drainage system that connects the two countries, 15 feet below the surface. Six Mexican national guardsmen wielding flashlights form the front line, tracking shovel-bearing drug traffickers through the seven-mile stretch of drainage and sewage lines on the Mexican side. They wonder out loud how many tunnels they haven't been able to find. "Nogales is the capital of cross-border tunnels between Mexico and the United States," said Ricardo Santana Velázquez, the Mexican consul general in Nogales's namesake on the U.S. side of the border. The subterranean challenge of stopping drug trafficking and human smuggling is on daily display here. Once traffickers discovered the Nogales drainage system, they learned to drill and hammer into the walls of the channel. Authorities don't know how much contraband makes it through the tunnels before they are found and blocked. But they say a narrow hand-dug passage could be worth tens of millions of dollars to drug cartels. On a scorching recent Monday afternoon, Santana and the team of Mexican national guardsmen crawled into the underground drainage system. Traffickers seemed to enter every day, drilling tunnels that bisect the sewer like holes in Swiss cheese, each of them connecting the pipeline to Arizona. One tunnel led to the bathroom of an Arizona home - and 211 pounds of cocaine and fentanyl, meth and heroin. Another opened up in an overgrown patch of grass just north...