The counter-insurgency on America’s southern border.
HOW MANY TUNNELS ARE THERE?
Authorities discovered 224 border tunnels originating in Mexico from 1990 to March 2016, including 185 that entered the United States, according to the latest U.S. Drug Enforcement Administration annual survey. Many are shallow holes, but some are elaborately constructed with hydraulic lifts, water pumps and rail cars.
The vast majority are in Arizona, where smugglers connect to underground drainage canals in Nogales, and in California, where construction noise generates less attention amid warehouses of an industrial area of San Diego, across from densely packed homes and businesses in Tijuana.
WHAT ARE THEY FOR?
Tunnels are generally used for multi-ton loads of marijuana because the drug's bulk and odor are difficult to conceal for motorists and pedestrians who enter the United States at official border crossings, the preferred method for smuggling methamphetamine and heroin.
In 2015, authorities seized cocaine in connection with two California tunnels, including one that ran underwater from a house in Mexicali, Mexico, to the All-American Canal near the city of Calexico.
HOW ARE TUNNELS FOUND?
Leads from informants, neighbors and others have been the most trusted technique, but technology plays a part. Lance LeNoir, who leads the Border Patrol's "tunnel rats" team in San Diego, says seismic devices, acoustics and ground-penetrating radar complement human intelligence.
© Gregory Bull A member of the Border Patrol's Border Tunnel Entry Team looks on from a tunnel entrance in between two border barriers separating San Diego and Tijuana, Mexico, in San Diego.
Border Patrol 'tunnel rats' plug underground passages