Police found 500 cannabis plants packed into two terraced houses in Leicester thanks to anonymous tip-offs from the public.
Officers said the criminals behind the two factories – within a short distance of each other in Leicester's West End – would be “royally upset” because the plants were ready to yield their first harvests.
The plants would have generated hundreds of thousands of pounds had the drug been allowed to reach the market.
The Leicester Mercury was given exclusive access to one of the illegal growing operations – a house in Paton Street, off Narborough Road.
Video footage taken by the Mercury gives an insight into the intricacies of cannabis factories, which have exploded in number across the UK in the past decade.
It shows mature plants packed into two rooms, while a smaller room serves as a nursery where young plants were cultivated.
It also shows the haphazard electrical wiring which powered the heating and ventilation systems required to cultivate the hundreds of plants.
The gangs had tapped into the mains to illegally abstract power.
The second house, in Harrow Road, was set up in the same fashion, although police at the moment have no evidence to link the two.
Officers raided the houses on Thursday and Friday last week after members of the public called the anonymous Crimestoppers hotline.
No arrests were made at the time and officers are continuing their inquiries this week.
Sergeant Simon Barnes, of Hinckley Road police station, said: “The criminals who set these things up will have invested a lot of time and money in these places.
“They were both at the same stage of growth, with the plants nearly ready for harvest, so we will have royally upset someone.
“We believe there are something like 250 plants in each house, so this is a fairly sizeable seizure and we have prevented a good amount of cannabis reaching the market.
“These are good finds for us, but we don't think the two are linked.
“It just happened we had two anonymous tip-offs to Crimestoppers at the same time and in the same area.
“I would urge anyone who thinks they have one of these places in their street to call us or Crimestoppers and we'll come along and check.”
A Harrow Road resident, who asked not to be named, said: “It's amazing these things can be set up in your street and nobody has an idea.
“How do they get all the equipment they need to get these things going into a house in a street like this without someone noticing?
“They are a notorious fire risk and I don't like to think about what could have happened if it had gone up in flames.”
Zuffar Haq, of Crimestoppers in Leicestershire and Rutland, said: “It's great to see results such as this from people calling in to us.
“It helps the police remove significant amounts of drugs from the streets of Leicester.
“It's important the police have been able to close down these places before they yielded their first harvests.
“The criminals who set them up would have spent a lot of money and time, perhaps months, and they will get no return on their investment.
“These places are also massive fire risks and, in cases such as these where the cannabis is being grown in terraced properties, there is a great danger of fires spreading to neighbouring houses.
CHECK OUT THIS LINK FOR VIDEO'S http://www.thisisleicestershire.co.uk/VIDEO-Cannabis-factories-raided-Leicester/story-19550889-detail/story.html#axzz2af8yUzpr