"Illicit cross-border tunnels allow smugglers to move marijuana and, to a lesser extent, other drugs, weapons, currency, people, and other contraband illegally across the border. More than 170 illicit crossborder tunnels have been discovered in the United States since 1990.53 These tunnels have been found near POEs, where traffic and noise conceal tunneling activities.
"CBP and U.S. Immigration and Customs Enforcement (ICE) have delineated four types of cross-border
tunnels. The type of tunnel constructed depends heavily on the region’s soil.54
"• Rudimentary tunnels are crudely constructed and travel a short distance. Shoring, machinery, electrical power, and ventilation are not used in their construction.
"• Interconnecting tunnels link at least one purpose-built section to preexisting underground infrastructure. The purpose-built section is usually crudely constructed.
"• Sophisticated tunnels may use shoring, ventilation, electricity, railroads, or water pumps, and can move large quantities of drugs, humans, currency, or firearms across the border. They typically link to private homes or warehouses in the United States and Mexico, even over long distances.
"• A fourth type of tunnel has been identified within the past five years in which traffickers employ horizontal directional-drilling equipment to construct a small-diameter tunnel in as little as two weeks. Attempts to construct these tunnels are infrequent.
"Marijuana comprises the overwhelming majority of illicit drug seizures from tunnels. Smuggling cocaine, heroin, and methamphetamine through POEs is easier than moving marijuana due to the relatively smaller size of shipments."


National Southwest Border Counternarcotics Strategy, Executive Office of the President, Office of National Drug Control Policy, Washington, DC. July 2016, p. 10.