Italian police have seized 340kg of cannabis from inside an abandoned metro station that was built in the 1930s by Mussolini.
Officers raided the half-a-mile long tunnel system in Rome, constructed during the fascist leader's time, after detecting the pungent aroma of marijuana near the entrance.
The haul they came up with is thought to be worth £2.4m and the fumes are said to have been so strong that officers were only able to stay in the tunnel for half an hour at a time.
Officially, the tunnel was being used a mushroom farm.
Underground passages that run near the Italian central bank's vaults in the southeast of the city are commonly used to grow mushrooms - but hidden behind one set of walls were the huge marijuana plants.
Video footage released by police revealed blocks in the wall swung aside to allow access to the 43,000sq ft tunnel.
Stacks of bags filled with cannabis were discovered along with plants, weighing and processing equipment. Overalls, boots and gas masks were also found inside.
It is one of the largest cannabis busts Italy has seen.
A 57-year-old who is thought to be the owner of the farm has been arrested.
Benito Mussolini had the tunnel built as part of a metro network in the 1930s, but work was abandoned after Italy joined World War Two in 1940.
According to Italian newspaper Corriere Della Sera, workers at the cannabis factory operated in shifts and changed clothes before leaving the tunnels. It claimed the operation may have been run by the Camorra, a violent mafia-style organised crime group based in Naples.