Police officers stormed an Exmouth business unit and recovered around £200,000 worth of cannabis plants.
Around six officers raided the premises at unit 1A on Pound Lane, previously the Lions Den Gym, after receiving reports about a strong smell coming from the building.
A warrant was immediately sought from Exeter Magistrates Court and executed, mid morning on Friday, January 11.
Officers discovered around 300 plants which were at various stages of cultivation in four rooms. Around 125 had recently been cropped.
With an average market value of £700 per tree, the plants, including those recently cropped amounted to around £200,000.
PC Jenny Palmer, investigating, said officers found a "sophisticated and professional" operation.
No arrests have yet been made and anyone with any information about who may be responsible are urged to contact the police.
Officers took around eight hours to dismantle the set-up and remove the plants for testing.
When officers arrived, because of the tampering with the electrical supply, for safety reasons Western Power engineers had to be called in to make the premises safe.
"When our intelligence officer made the initial investigation it was a frosty morning and he noticed that part of the roof had no front on it, suggesting a significant heat source," explained PC Palmer.
"He could also smell the strong cannabis odour."
PC Palmer said officers found an extensive system with four to five inch thick insulation on the walls and a sophisticated watering system with hoses going to each plant.
"To find an operation of this scale and value in Exmouth is an unusual occurrence," she continued.
"We know that the current tenants have been renting the unit since August. We would like to hear from anyone who may have any information about people coming and going from the premises, to contact us as it may help us with our enquiries."
Police can be contacted on 101, or anonymously via Crimestoppers by calling, 0800 555111, quoting crime reference number, KE13/91.