Four Vietnamese nationals have been released from custody by UK police after they were found working at a cannabis farm in the southwest of England.

Police found three teenagers aged believed to be about 19 years old, and an adult man in his thirties.

All are believed to be victims of human trafficking.

British newspaper The Guardian reported that the four men were found during a midnight raid.

They were working in a nuclear bunker with no natural light, fresh air or running water.

Police found several thousand plants in the bunker estimated to be worth over $1.5 million dollars.

Police are investigating how the men came to be in the bunker, though no further action is being taken against them.

Last year, UK police discovered a number of large cannabis operations including one at a former bank, another at a disused sports centre in Wales, and a recently emptied GP surgery in the country’s north west.