An elaborate £1m cannabis farm has been uncovered in a former mill building complete with living quarters and a lift disguised by a stone fire place.
Police had to break through breeze-block walls built to hide the three-floor farm in Darwen, Lancashire.
They found 1,300 plants with a value of at least £760,000, 16kg of loose cannabis worth £230,000 and £250,000 of growing equipment.
A 31-year-old man was arrested at the premises on Wood Street.
Det Insp Vinny De Curtis said the operation was "enormous" and "one of the most sophisticated in the UK".
On first entering the property officers were met with "nothing more than empty rooms," a Lancashire Police spokesman said.
But after they broke through breeze-block walls the full extent of the operation was revealed.
It incorporated nearly all of the building's 20 rooms, including living space for "long term residence" and by-passed mains electricity, the police spokesman said.
Some areas had been built with the "sole intention of disguising their existence".
Det Insp De Curtis said: "This is the largest and most sophisticated cannabis factory I've ever experienced.
"I am delighted that we have been able to take such a large amount of illicit drugs off the streets of Lancashire before they are able to reach vulnerable people within our communities."
The farm was uncovered in a joint operation which also involved staff from Lancashire Fire and Rescue and the UK Border Agency.
The arrested man, from Darwen, was held on suspicion of cannabis cultivation and released until 30 August.