Police in southwest England uncovered an “enormous” cannabis factory during a midnight raid on a Cold War-era nuclear bunker.

Officers discovered thousands of plants worth an estimated $1.2 million inside the Chilmark Regional Government Headquarters’ underground shelter in Wiltshire, on Wednesday night.

“There are approximately 20 rooms in the building, split over two floors, each 200 feet long and 70 feet wide,” said Wiltshire Police detective inspector Paul Franklin via a statement. “Almost every single room had been converted for the wholesale production of cannabis plants.”

There was also “a large amount of evidence of previous crops,” he added.

Police acted on a tipoff to stake out the “almost completely impenetrable” shelter, which they said was constructed in the 1980s to house and protect government officials in the event of a nuclear attack.

On seeing three men leave the former Ministry of Defense-owned site, they arrested them and seized their keys. Officers then found and arrested two more men and a teenage boy inside, who they believe were tending to the cannabis plants.

Police raid nuclear bunker and discover county’s biggest cannabis factory. Read the full story on our website: https://t.co/RxwaJtMgne pic.twitter.com/nQ7ndyDSIM — Wiltshire Police (@wiltshirepolice) February 23, 2017

Franklin described the setup as the “largest and most sophisticated cannabis factory I’ve ever experienced.”

The three men detained outside the bunker were aged 27, 30 and 45 and from the neighboring county of Somerset. Officers arrested each of the trio on suspicion of cannabis production and human trafficking.

The three arrested inside, aged 15, 19 and 37, who were of no fixed abode, face cannabis production charges.