Mexican marines on Friday said they had discovered an underground drug lab in the mountains outside the capital of Sinaloa state where they destroyed 50 tons of methamphetamine.

Marines found the lab after intelligence reports indicated that tons of drugs were being produced in the area of Alcoyonqui municipality, about 12 miles outside the state capital of Culiacan, the marine ministry said in a statement.

The drugs were hidden underground in two areas along with barrels of the chemicals used to produce meth. The drugs were incinerated on site due to the difficult access to the remote location, the ministry said.

Mexico is a major supplier of methamphetamine to the United States and is the top source of heroin, which is fueling a resurgence in opioid addiction worse than the U.S. seen in the 1960s and 1970s. The country is also the principal highway for cocaine trafficked north.

Sinaloa is the seat of the powerful cartel formerly run by Joaquin “El Chapo” Guzman, who is facing trial in the United States later this year.