Police have discovered Hertfordshire's biggest ever cannabis factory - with 1,300 plants and seven growing rooms.
The factory was found in a unit at Woodside Industrial Park in Letchworth Garden City after a tip-off from members of the public.
The value of the annual crop is thought to be around £1.5m.
Police made the discovery during a 24-hour operation. It was concealed behind false walls alongside a living area and drying room.
They have since been destroyed.
Ba-van Nguyen, 20, of no fixed address, has been arrested and charged with being concerned in the production of a controlled drug.
A 16-year-old boy was also arrested and charged with being concerned in the production of a controlled drug.
Police say the public should look out for the following signs of cannabis being grown in communities:
- A strong and sickly sweet smell
- Lighting and ventilation equipment being taken into the house
- Windows always covered or blocked off
- People coming and going at all hours or neighbours you never see
- Strong and constant lighting at all hours
- High levels of condensation
- Constant buzz of ventilation equipment
- Lots of power cables around the property