Otay Mesa sits on a plateau that crosses the US-Mexico border. The ground there "is made of clay called bentonite that is self-supporting and as workable as wax."
A Homeland Security agent enters a tunnel used to smuggle drugs between the US and Mexico in this photo released on October 31, 2013, by the US Immigration and Customs Enforcement Department (ICE). The tunnel linked an industrial park in Otay Mesa, California, with Tijuana, Mexico, and featured both rail and ventilation systems.