A Nogales (Ariz.) Police officer initiated the foot pursuit that eventually led to the discovery of one of two smuggling tunnels, an arrest and seizure of more than 800 pounds of marijuana worth more than $400,000, Chief Jeffrey Kirkham tells POLICE Magazine.

After encountering three suspicious people coming out of the Nogales wash area on the evening of Sept. 29, the Nogales officer attempted to contact them. When they fled, the officer chased them.

Additional assistance from Nogales officers and Border Patrol agents led to the seizure of the marijuana. Officers interviewed suspects who led them to the tunnel that used a storm drain at Nelson and International streets.

The tunnel was at the corner about 25 feet north of the border wall and 500 feet east from the foot traffic port of entry. The tunnel, which tapped into the Nogales drainage system, originated from a house in Mexico that has been the source of several prior tunnels. A steel grate opening on the street was used as an exit point on the U.S. side, Kirkham wrote in an e-mail to POLICE.

"There appears to be little cooperation to tear down these vacant dwellings and eliminate them from the area as a point of tunnel origination by the Mexican government," Kirkham wrote about the source house.

A vast network of hundreds of tunnels forms an underground netherworld that enables smuggling of drugs and people from the higher elevations on the Mexico side of the border.

"Nogales-Sonora, Mexico has a higher elevation than most of Nogales, Ariz., [and] as such it provides the terrain perfect to tunnel without risk until one gets to the lower elevation on the U.S. side of the border," Kirkham added. "The U.S. side of the border has engineered storm and sewer systems that can easily be tapped into. Many of the tunnels are dug to tie into existing U.S. drainage systems, thereby reducing the need to dig additional lengths of terrain. The water flow in the area is one that flows north thereby creating issues in itself when it rains."

The tunnel network is troubling to city works officials who say it has weakened the terrain and has put buildings at risk of collapse, reports the New York Times.

Tunnels that are discovered are sealed with concrete after assessments of origin, potential uses, and intelligence information has been gathered.

For pictures of the tunnels, view the New York Times coverage of the tunnel discovery.