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The facility in Huerta de Valdecarábanos, Toledo, Spain, spanned 120,000 sq metres - more than 16 standard football fields - and was supposedly growing hemp for industrial use.
But when police from the Civil Guard inspected the farm, they found that it was actually growing Cannabis sativa, genetically modified to look like legal hemp.
On their return, officers seized 135,000 marijuana plants - the largest seizure ever made during a police operation in Europe - and made three arrests.
In nearby warehouses, they intercepted a further 30 tonnes of dried marijuana and 3,720kg of buds that were ready for distribution.
At Madrid's main airport, another 112kg of buds - declared to be simply 'plants' - had already been seized by police.
A Civil Guard spokesperson described the enormity of the drug-growing operation.
They said: "The farm where this plantation was located covered an area of 12 hectares, with permanent surveillance 24 hours a day.
"In the vicinity there were two warehouses arranged with material to be converted into indoor marijuana crops for a greater increase in production.
"There was also a drying area with a closed system of security and surveillance cameras."
The spokesperson added that the workers at the farm were mostly foreigners, employed in an unofficial manner and housed in a place that did not meet 'the most minimal conditions of habitability'.
Three people have now been arrested for a suspected crime against public health, while another six are under investigation.
The raid comes amid growing fears about the scale of the drug trade in Spain.
One Civil Guard police chief, speaking in 2019, said: "Due to its geographical position, Spain is a receiver - hashish from Morocco and cocaine from Latin America. It's also a big marijuana producer.
"It has perfect, fluid distribution channels, mainly road transportation, with trucks carrying all kinds of fruits and vegetables across Europe."
A report from last year, meanwhile, singled out Catalonia as 'the epicentre of Europe's illegal marijuana market'.
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