Police have uncovered a 670m (2,200ft) drug smugglers' tunnel under the US-Mexico border - the second such discovery this month.
The tunnel links the Mexican city of Tijuana with Otay Mesa in California.
Earlier this month a tunnel equipped with ventilation and lights was found. Local media say the new tunnel may be even be more sophisticated.
The authorities say the tunnels are used to smuggle marijuana into the US, bypassing stringent border controls.
Investigators with the San Diego Tunnel Task Force said the latest tunnel led from a house in Tijuana to a warehouse in Otay Mesa, California.
Several suspects were arrested both in Mexico and the US as part of the operation.
The Associated Press reported that more than 20 tons of marijuana was seized in connection with the cross-border tunnel.
US Customs official Mike Unzueta told AP that the tunnel was believed to be the work of Mexico's Sinaloa cartel.
Earlier this month, border police found more than 20 tons of marijuana during an operation centred on a smugglers' tunnel running along a similar route to that found on Friday.