An investigation into several hemp fields in California uncovered 10 million illicit cannabis plants with a “black market” value of over $1 billion, which authorities seized and destroyed, officials said Thursday.

The investigators searched 11 different fields in the Arvin area, located about 100 miles north of Los Angeles, on Oct. 25 after obtaining a search warrant, the Kern County Sheriff’s Office wrote on Facebook. They swept through 459 acres of land with assistance from the California Department of Fish and Wildlife (CDFW) and FBI.

The plants were grown under the appearance of a legitimate hemp operation, the sheriff’s office said. Samples of the plants were tested to determine their level of THC, the psychoactive ingredient in marijuana. The levels tested “well above” the legal 0.3 percent limit for industrial hemp production, essentially making the plants cannabis, authorities said.