Since 1995 more than 90 illicit underground passageways have been discovered in various states of completion in the two-mile stretch of urban frontier that separates Nogales, Ariz., from its twin city on the Mexican side of the border. Twenty-two complete tunnels have been found in the past three years alone. The city has become infamous as the Tunnel Capital of the Southwest.

Pictured, the 10-inch-wide entrance to a smuggler’s tunnel uncovered in August 2011.