More than 1,000 plants have been found in three separate cannabis factories in the past week.
The three factories in Rhondda Cynon Taf were all found in large disused buildings after police received tip offs from members of the public.
The first factory was discovered on February 27 inside the old Diamond Jubilee Hotel building in Tylortsown, Rhondda .
Officers from South Wales Police has been guarding the property since the discovery, unable to enter the premises due to the building's "structural integrity".
Work has had to be carried out on the first and second floor of the property in order to allow police officers to enter.
The number of plants inside the property is unknown, but a spokesman for South Wales Police said plants were found inside a number of rooms in the large building.
This isn't the first time the building was targeted for this use, in 2007, £2m worth of cannabis plants were discovered there.
A police spokesman added: "Less than a week later, on March 3, a large cannabis factory was discovered inside the old NatWest bank building on Forge Street, Pentre.
"And most recently, on Wednesday March 4, the old Bingo Hall on High Street, Pontypridd, was found to contain a number of plants."
A 26-year-old man has been arrested in connection with the discovery in Pontypridd, and is currently in police custody.
Though there is no exact number on how many plants were discovered in each of the properties at this time, the spokesman said police "have seized a combined number of over 1,000 plants".
If you have any information you think could assist officers in relation to the discoveries call 101, quoting reference 072423.
