The Mexican government says troops have discovered another drug smuggling tunnel beneath the U.S.- Mexico border and seized more than 40 tons of marijuana. The tunnel, beneath the Tijuana (Mexico) – San Diego (California) border was the fourth such discovery in the past week along the 3,200 kilometer boundary separating the two countries.

Authorities say three of the tunnels were still under construction, including a 365-meter passage that began in a Tijuana warehouse where the marijuana was seized, and extended 100 meters into the U.S. side. All four passages, including one found in Arizona, were equipped with lighting and ventilation. One of the digs included a rail transport system.

Mexican and U.S. authorities on Thursday said the 220-meter tunnel linking San Luis, Arizona and San Luis Colorado on the Mexican side showed a high level of engineering expertise.

The U.S. Drug Enforcement Administration said the San Luis passageway is the only known completed and fully-operational smuggling tunnel ever discovered in the area near Yuma, Arizona.

U.S. authorities say 89 cross-border tunnels have been discovered in Arizona and 50 others in California in the past decade, as agents on both sides of the border press to curtail the flow of narcotics into the United States.