The ground in the area around Otay Mesa has proven especially impenetrable for law-enforcement detection methods. "Most deep-set geophysical variations—seams of rock, mismatched strata of soil, and excavation projects—show up as indistinct lines," on the screens of the ground-penetrating radar machines that authorities use. To investigate each suspicious blip would cost too much.
A law-enforcement officer enters a drug tunnel found in a warehouse in Otay Mesa, California, April 3, 2014, in this handout photo released on April 4, 2014. US federal agents uncovered two drug-smuggling tunnels underneath the US-Mexico border, both surfacing in San Diego-area warehouses and equipped with rail systems for moving contraband, officials said. The discovery led to the arrest of a 73-year-old woman accused of running one of the warehouses connected to a drug-smuggling operation, according to a joint news release by four federal agencies.