The Border Patrol keeps watch on this Arizona town with cameras, patrol cars and agents on foot in order to prevent the illegal entry of undocumented immigrants and drugs, so that traffickers have switched to sending their goods underground. Their 100 underground crossings make Nogales the narco-tunnel capital of the world.

In a town of almost 21,000 inhabitants, there are abandoned stores and houses very close to the border wall, which facilitates illegal excavations to bring drugs from the neighboring town of Nogales, Mexico.

"What happens is that there are underground drainage systems in both of the Nogales cities, so trafficking organizations smuggle drugs through those drains to bring their contraband" into the U.S., Border Patrol spokesman Andres Adame said.

Authorities have found narco-tunnels under residential and commercial neighborhoods and public parking, which forced the city to remove all the parking meters that were on International street beside the border wall.

"What they were doing was to park a car with no floor over an entry to the drains, crossed the packages over the border, put the cement cover back in place, and - you won't believe this - they had a hydraulic jack," Nogales Mayor Arturo Garino told Efe.

Despite everything, residents say Nogales is a calm, peaceful place to live with picturesque homes and parks, as well as an active business district where visitors from the Mexican state of Sonora come every day to do their shopping on the U.S. side.

Residents say sarcastically that there is so much tunneling under the city that there could be a massive sinkhole.

"No, I don't believe it's going to sink, because most of the tunnels that have been discovered are not in the business district, they're at the sides," Mayor Garino said.

Nonetheless, Border Patrol agent Nicole Ballistrera showed us a street where the sinkage has already begun.

"There's a tunnel here. A car was driving down this street and it suddenly fell because underneath they were digging a tunnel," the agent said, pointing to where the sinkhole had been filled with cement. EFE