More than 1,000 cannabis plants have been discovered in a disused pub in Highgate, Birmingham.
The offenders sometimes stole electricity to grow the plants
West Midlands Police said the find, during a raid on Tuesday afternoon, was one of the region's biggest seizures.
The plants were being grown under artificial light, often using power stolen from the National Grid.
The profits from the drugs produced by the plants could be well over £100,000 every eight weeks, officers said. Two men have been arrested.
Police warned there could be as many as 300 similar factories operating secretly across the region.