Italian police alerted by a powerful tell-tale odour discovered a marijuana farm worth about €3-million in an abandoned Mussolini-era tunnel in the capital Rome, a police spokesperson said on Wednesday.
The police arrested a 57-year-old man and were looking for other suspects after seizing 340kg of the drug grown in rooms lit by halogen lamps and fed by an extensive underground irrigation system.
A special financial police unit carrying out a patrol on Saturday "was alerted by the strong odour of marijuana coming from the air vents" of the tunnel near the Rome-Casilina train station, a police spokesperson told AFP.
The police, who had to put on masks, found "a 4000 square metre area where greenhouses were set up for growing and producing marijuana," the financial police said in a statement on its website.
Police had first discovered what appeared to be a mushroom farm before finding a moveable brick wall hiding the marijuana plantation, the spokesperson said.
In a storage area, police found black garbage bags filled with newly cut plants and small bags with the finished product. Also found were machines for drying the plants, scales and chemical products for preparing the cannabis.
The tunnel was part of an ambitious project for the underground train line launched in the 1930s by the fascist dictator Benito Mussolini, which was later abandoned.
The area located in the working-class neighbourhood of Tor Pignattara is home to a number of underground mushroom farms because of moist conditions and stable temperatures.