LONDON (Reuters) – Three Britons were jailed on Friday for running a huge cannabis farm inside a former Cold War nuclear bunker in an isolated area of southwest England.

The gang used the bunker, built in the 1980s to house local government officials in the event of a nuclear attack, to cultivate more than 4,000 cannabis plants capable of producing 2 million pounds ($2.6 million) worth of drugs a year.

The bunker, located in Chilmark near Salisbury, was no longer owned by the Ministry of Defence but was still intact with its nuclear blast doors in place which made it almost impenetrable, police said.

The men, who were arrested in a midnight raid in February but only after they had left the site, also locked workers inside so they could tend to the plants 24 hours a day.

The farm was thought to have been running since 2013. To keep it operating, the gang bypassed the mains supply and instead siphoned power from a pylon outside, illegally using about 650,000 pounds worth of electricity.

“This was without doubt the largest cannabis factory we have seen here in the county, with almost all of the 20 rooms inside the bunker converted for the wholesale production of cannabis,” said Detective Inspector Simon Pope of Wiltshire Police in a statement outside the court in Salisbury.

The men pleaded guilty to conspiracy to produce class B drugs and conspiracy to abstract electricity.

Martin Fillery, 45, was jailed for eight years and his accomplices Ross Winter, 30, and Plamen Nguyen, 27, for five years each.