A £200,000 cannabis farm has been discovered hidden in a bunker 10ft beneath a repair garage in Knuzden.
Police discovered around 500 plants in two shipping containers at the premises on Haslingden Old Road yesterday afternoon.
Equipment, worth an estimated £10,000 was also seized from the unit.
Four men, aged 16, 31, 41 and 43 were arrested on suspicion of producing cannabis and are being questioned by the police.
DI Neil Ashton said police raided the premises on Wednesday, February 29 after receiving an anonymous tip-off two days earlier.
He said: "When we arrived, we found two huge shipping containers which had been buried behind the wall and under the soil. We managed to gain access through a hole in the wall which was hidden behind some shelving – it was a very professional set up.
"I have no doubt this will have a significant impact on the supply of cannabis across East Lancashire."
He added: "We are committed to acting on information to remove drugs from our communities and I urge local residents to report their suspicions so we can take action."
Anyone with information or concerns about drugs can contact police on 101 or Crimestoppers anonymously on 0800 555 111.