Customs officials are investigating after two tunnels used to smuggle drugs from Mexico into the US were discovered.
The cross-border tunnels - housed in warehouses in San Diego's Otay Mesa industrial area - were used to smuggle huge amounts of marijuana from Tijuana, Mexico.
They run for about 2,000 feet (600 metres) and were equipped with rail car, lighting and ventilation systems.
According to US authorities, both were the work of Mexico's Sinaloa cartel, headed by that country's most-wanted drug lord, Joaquin "El Chapo" Guzman.
The discoveries produced some of the largest marijuana seizures in the United States.
Immigration and Customs Enforcement agent Tim Durst said: "To an extent this is somewhat of a cat and mouse game. As long as there's demand for the supply of drugs in the United States then the drug smuggling organisations will continue to find a way to smuggle drugs into the United States."
He added: "They're looking to find new ways to move drugs and we're looking to find the ways that they're moving drugs. Their hope is they get a tunnel completed, they can move a lot of drugs into the United States and get a good return on their investment.
"Our hope is we find tunnels or the boats or whatever next they're next going to before they get a return on their investment."