Mexican police reported finding an underground tunnel used to smuggle migrants into the U.S., suggesting that criminal organizations are finding new uses for these "narco-tunnels," which are normally used to transport drugs and weapons.

Officials said they discovered the tunnel near the northern border town of Nogales, Sonora. It had been dug into a storm drain, with a hidden exit on the U.S. side. The Public Security Ministry (Secretaria de Seguridad Publica Federal - SSPF) reported that the find was the result of an investigation carried out after securing another tunnel in the area on August 16.

So-called "narco-tunnels" have long been a feature of the cross-border drug trade. El Universal reported recently that more than 150 have been shut down since 1999. But now border officials are concerned by evidence suggesting that criminal groups are increasingly relying on the tunnels in response to tougher security measures at checkpoints. As the Arizona Republic reports, agents in the U.S. Border Patrol's Tucson Sector discovered eight tunnels in the first seven months of this year. According to Tucson Border Patrol spokesman Mario Escalante, they found only five in the same period in 2010.

The trend has been met with a degree of self-congratulation, as officials claim that it is due to their success at monitoring the flow of drugs and other illicit goods at the border. "As smuggling organizations have more trouble moving their contraband both between the ports of entry and through the ports of entry due to increased technology and vigilance at the ports, then they will turn to more of these covert measures," Vincent Picard, a spokesman for Immigration and Customs Enforcement in Phoenix, told the AP recently.

This analysis is likely correct; as InSight Crime has noted, tightened U.S. border security in the wake of September 11, 2001, has driven groups of migrants to take increasingly remote routes to cross into the country. This has resulted in a higher death toll, as the migrants must trek through harsh, desert regions rather than sneaking over the border closer to more populated areas. It also means that migrants require guides for the difficult terrain, professionalizing the border crossing business and thus drawing in organized criminal groups. This shift may have caused migrant traffickers to invest in tunnels, as a better way of getting their human cargo across the border.

If criminals are now using tunnels to smuggle entire groups of migrants, rather than just to ship drugs, this may also be related to improvements in tunnel sophistication. In May, officials uncovered a 250-foot long tunnel in the Nogales area, much of which had been carved out through solid rock solid rock. Because of its sophistication (it had electricity, lighting, air conditioning and water pumps), the discovery made international headlines. But U.S. and Mexican police have been seizing the tunnels for years now.

Whereas in the past the tunnels were constructed by small teams of laborers, it is now common for the tunnels to be designed by professional architects and engineers. They are frequently equipped with reinforced concrete walls, air conditioning, telephone lines and surveillance cameras. Indeed, El Universal cited a Northern Command report as saying that some tunnels constructed by the Sinaloa, Juarez and Gulf Cartels have been valued at one million dollars.

Traditionally the tunnels are constructed in Mexico, where there are less resources to monitor the large-scale machinery use required to construct them. However, this too has begun to change recently. On August 23, police called to a house in Douglas, Arizona, were shocked to find several of the rooms filled with dirt and debris -- and one with a large hole in the floor.