NOGALES, Ariz. -- Drone aircraft patrol the U.S.-Mexico border from the skies. Fast boats look out for smugglers at sea. And tens of thousands of Border Patrol agents use trucks, horses, all-terrain vehicles and bicycles to stop unauthorized crossers on land.

But there is another route across the border, one in which smugglers slither north. As enforcement efforts have increased and border barriers have been built, tunneling has gained in popularity, with Nogales becoming the capital.

On Thursday, the Border Patrol was filling an underground tunnel that had been discovered right under the immigration checkpoint in Nogales. But even before the concrete was poured to make that tunnel inoperative, another subterranean passageway was discovered a block away.

The second tunnel, which had been used to bring bales of marijuana from Mexico, will be filled as well. There are patches, in fact, all across this city, where the authorities have tried to tap the tunnels that traffickers build off the extensive underground storm drain system that connects Nogales with another city by the same name across the fence in Mexico.

With profit margins so huge, drug traffickers pushing their wares across the border are an enterprising lot. No matter how much the U.S. government pours into the region to stop them, there always seem to be novel attempts to elude detection.

And the two Nogaleses are where drug trafficking has literally gone underground. Burrowing from one country to the other happens elsewhere along the border, particularly in the smuggling zone around Tijuana. But officials say most of the tunnels discovered along the entire stretch of the border are from the Mexican Nogales to the American one.

"We are in the lead in the tunnel business," said Chief Jeffrey Kirkham of the Nogales Police Department.

It is the geography of the region that makes tunneling so common here, as the Mexican side sits at a slightly higher elevation and water flows north through generations-old underground channels. "Through the downtowns of both cities, the drainage flows through a tunnel and then at some point goes into an open channel on the U.S. side," said Sally Spener, spokeswoman for the International Boundary and Water Commission, a binational body that oversees water issues between the United States and Mexico.

Over the last four years, at least 51 unauthorized tunnels, or more than one a month, have been found in the two border cities.

Some are short, narrow passageways that require those navigating them to slither. Others are long, sophisticated underground thoroughfares strung with electric cables and ventilation hoses.

Last year, a resident tipped off the authorities to a tunnel that extended 48 feet into Mexico and 35 feet into the United States, making it one of the longest ever found in Nogales.

One high-end tunnel found in 2005 farther west in Calexico, Calif., originated in the master bedroom of a Mexican home and extended to a garage on the American side. It had a phone line and air conditioning, and the authorities estimated that dozens of truckloads of dirt had to be removed to build it.

Although migrants heading north sometimes use tunnels, the passageways are more often considered the handiwork of drug smugglers. That means residents, especially on the Mexican side, sometimes look the other way when they observe surreptitious tunneling for fear of attracting the attention of criminals.

On the Arizona side, specially trained Border Patrol agents monitor the drains, entering the dark underworld that crisscrosses the border and looking for unauthorized offshoots dug by hand.

The air is cool down below. The only sound comes from the chirping of bats and the flow of water, a mixture of storm runoff and sewage. It seems a good place to hide.

"I'm one foot from the border," Kevin Hecht, a Border Patrol agent standing in the dark in a stream of pungent water, said as he shined his flashlight around. "Down here, you look for signs of movement. You look for digging."

Farther down the drain, David Jimarez, a Border Patrol spokesman, squatted in a tunnel and peered into a two-foot offshoot. "They crawl on their bellies," he said. "They're like a snake."

How the tunnels are discovered varies. The one filled with concrete last week was found when the front tire of a bus sank into the pavement, revealing a weak spot that was caused by tunneling. There have been cases, officials say, of manholes popping up in the middle of roadways, with furtive eyes peering out. The owner of a Nogales warehouse last year discovered a tunnel in his border-front property.

Sometimes the diggers make too much noise. In June, a security guard at the DeConcini Port of Entry reported hearing strange sounds emanating from a storm drain that ran from the border fence north to Interstate 19. It turned out to be a tunnel just large enough to fit a smuggler.

"It's a netherworld down there," said Roy Bermudes, the assistant police chief in Nogales. "If you turn off your flashlight, you can't see your hand in front of your face."

Bermudes used to enter the tunnels regularly when he led the police SWAT team that provided backup to city workers doing underground repair work. He recalls hearing a noise while underground and aiming his rifle in front of him, only to discover a Mexican military squad doing a similar patrol. After a brief standoff, the guns on both sides were lowered.

Eventually, as the danger grew, the city handed over patrol duties to the Border Patrol, which has installed underground cameras and motion detectors.

It is not just the flow of drugs that concerns the authorities here. The tunneling weakens roadways, sometimes causing them to buckle, and puts buildings at risk.

"There is a joke in Nogales that someday its entire downtown will collapse into a giant sinkhole due to the many drug tunnels in the city," Hugh Holub, a former public works director in Nogales, wrote recently