Up to 3,000 cannabis plants which police say have a street value of around £1m have been found in a former social club in Swansea.
Officers say the drugs factory in the Four Seasons Club in Llansamlet is the biggest uncovered in the city.
The council-owned building is understood to have been empty since earlier this summer.
South Wales Police said its investigation was in its early stages but five people had been arrested.
Four Vietnamese males aged 27, 26, 16 and 14 together with a 52-year-old local man are in custody at Swansea Central police station.
Police executed a search warrant under the Misuse of Drugs Act on Tuesday.
They discovered that the main function room together with a number of side rooms at the club housed a fully functioning cannabis plantation with 2,500 to 3,000 plants.
The rooms were fitted with heating and lighting equipment along with a sophisticated hydroponics system.
Det Ch Insp Peter Azzopardi, who is leading the investigation, said: "This operation has come to fruition as a result of community information which we have acted upon.
"This is by far the largest cannabis plantation we have come across within our area and considerable resources have been and continue to be involved in this investigation.
"I am appealing to anyone who may have information in respect of this matter to come forward, either directly or via Crimestoppers.
"In particular we are seeking information in respect of the Four Seasons Club which we believe has been closed since mid-July 2010.
"We are seeking any information in respect of activity and movements at the premises since this date".
Local councillor June Evans said people would be "shocked" following the discovery.
She said the club was once "the hub" of the community with bingo, shows, meetings and more recently it was used a gym.
"Over the years it was the place to go. It was very popular but unfortunately it has been run down lately," she said.
"I think people will be shocked to find out what's been happing here."
Contact police in Swansea on 101 or independent charity Crimestoppers on 0800 555111.