Thousands of cannabis plants with a street value of more than £1million were seized by police in Watford.
The cannabis factory on the Shakespeare Road industrial estate was raided on Thursday after information was received from members of the local community.
More than 1,400 plants were being cultivated in two warehouses. Ten kilograms of bagged cannabis was also found and seized.
Two men were found at the site and are being treated as victims of modern slavery.
Herts police say they are continuing to trace those behind setting up the factory and cultivating the plants.
Sergeant Dave Perkins, who led the operation, said: “We are very pleased that we have prevented such a large amount of drugs from reaching the streets. The offenders had also illegally bypassed the electricity supply when setting up the factory, creating a potential fire hazard and putting innocent lives in danger.
“Drug dealing and the associated criminality can directly affect the lives of those in the vicinity and I want to reassure residents that we will always act on intelligence that suggests drugs are being cultivated.
“This seizure is a direct result of intelligence we received from the local community who reported their suspicions to us. The information we receive from members of the public is absolutely vital in helping us to tackle drug crime in Hertfordshire.
“Any information you share with us, however seemingly insignificant, could be the missing piece we need to make an arrest.”