DEA agents found a drug tunnel in a one-story building in San Luis, Arizona, near Yuma, July 7, 2012.

The San Luis tunnel was found after local police stopped a truck that was carrying 29 pounds of methamphetamine. The truck was traced back to the one-story building near the border, which the DEA had been monitoring.

DEA agents found a drug tunnel in a one-story building in San Luis, Arizona, July 7, 2012.

DEA agents found drums next to the opening that were full of the dirt that had been excavated from the tunnel.

Inside the nondescript white building, investigators discovered the entrance to the passageway. From the warehouse floor, the tunnel plunges more than 20 feet to the bottom of the shaft.

The passageway, measuring approximately four feet by three feet, is equipped with structural supports, electricity and ventilation. Evidence found inside the warehouse leads investigators to believe the tunnel was only recently completed.

This underground passageway was three-feet wide by five feet tall and originated from an abandoned building in Nogales, Sonora.

Border patrol officials described the tunnel as sophisticated reporting it had electricity, water pumps and ventilation.

Since 1990, law enforcement has found more than 115 tunnels underneath the U.S.-Mexico border. The tunnels are used for smuggling drugs, money, weapons and human beings. Only 11 of the tunnels were discovered before 9/11. Most have been discovered since 2001, after the U.S. stepped up border security. The tunnel shown here was found on March 12 in Tijuana southeast of the Otay Mesa border checkpoint.

U.S. authorities have formed a joint operation called the San Diego Tunnel Task Force, drawing agents from ICE, DEA, Border Patrol, NCIS and California's Bureau of Narcotics Enforcement, to uncover tunnels beneath the border between San Diego and the neighboring Mexican metropolis of Tijuana. American officials say their work has been made easier lately by better cooperation from Mexican law enforcement. As recently as 2006, said Timothy Durst, assistant special agent-in-charge of ICE, San Diego, "it was really tough working with Mexico. Now they are finding tunnels on their own." The tunnel shown here, found in March, was uncovered by Mexican law enforcement after the ground near a Tijuana warehouse collapsed.

Agents from ICE, DEA and the Border Patrol found this tunnel in Otay Mesa, Calif. in January 2006. The U.S. entrance, shown here, was found inside a warehouse.

Agents said the tunnel linked a warehouse in Otay Mesa, Calif. with a warehouse in Tijuana, Mexico.

Mexican authorities, acting on a tip, discovered an incomplete tunnel on November 7, 2009 inside a two-story home in Tijuana. The 401-foot tunnel did not cross the border, but was equipped with ventilation, electricity, a water pump and a dumbwaiter-style vertical lift.

The dumbwaiter-like elevator in the Tijuana tunnel was controlled electronically and extended 35 feet from the tunnel entrance inside a private home to the main tunnel.

Acting on information from the San Diego Tunnel Task Force, Mexican authorities arrested a dozen people inside a 1,000-foot-long cross-border tunnel in December 2009. The tunnel had been under construction for two years. The Tijuana entrance is seen here.

The tunnel included lighting, electricity, a ventilation system and an elevator on the Mexican side. Sophisticated tunnels are most often the work of drug cartels, which can spend up to $5 million on materials and labor, and are more likely to be seen in the San Diego area than in Arizona. The tunnels can take up to five years to build, said Durst. "They're using the same rudimentary tools, picks and jackhammers as 10 years ago."

The passageway was deep, reaching 100 feet under the surface, and extended 860 feet into the U.S., but diggers had not yet opened an entrance on the U.S. side of the border.

Another view of the unfinished Tijuana tunnel, with wiring and ventilation visible. When U.S. authorities are finished investigating a tunnel, Border Patrol agents fill it with concrete slurry, but only up to the Mexican line. "It's very costly," said Durst, " and the Mexican side can't afford to do it."

Inside a home on North Escalada Drive in Nogales, Arizona, agents from ICE and DEA found the U.S. entrance of a tunnel that stretched 100 yards to a house in Nogales, Mexico. The U.S. entrance was hidden in the utility room. Picks, a jackhammer and other equipment were scattered on the floor. U.S. officials say that Arizona has more tunnels than California, but that tunnels under the Arizona border are often cruder than those in the heavily populated San Diego/Tijuana area. These rudimentary passageways are sometimes called "gopher holes."

The Nogales tunnel measured only three feet by three feet at its entrance in Arizona. The tunnel had lighting, but no ventilation. Authorities believe the tunnel had not yet been used. Mexican officials arrested five suspects at the Mexican entrance.

Mexican civil engineer Luis Carlos Ayala-Gonzalez was arrested in 2008 for his alleged role in building a tunnel under a house in San Luis, Arizona. ICE agents found plastic tubing, drilling equipment and journals describing the construction of a tunnel. The tunnel was discovered by a Border Patrol employee who saw cement coming out of a ventilation hole. In November 2009, Ayala-Gonzalez was convicted of one count of aggravated assisting in a criminal syndicate and sentenced to three years in an Arizona state prison.

The San Luis tunnel extended from a residence on the Mexican side of the border to the basement of this house at 1429 San Francisco Street.