More than 240 weed plants stretching the width of the road have been found growing under the M1.
The bizarre cannabis factory has now been raided with the operation stretching across both carriageways.
The setup was discovered inside a drainage culvert, which had been boarded up to secretly grow the plants.
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The factory was located under the M1 near the village of Watford.
Eagle-eyed National Highways maintenance workers spotted the operation on 28 Nov, as reported by Need To Know.
Northamptonshire police were informed of the factory.
The Daventry Neighbourhood policing team set about dismantling it.
The operation was a treacherous one due to heavy rain – however they were assisted by a nearby landowner who provided transport and trailers.
At this moment in time no arrests have been made in connection to the cannabis factory.
Members of the public have been urged to contact the police if they have any information on the operation.
Jonathan Davis-Lyons, Daventry Neighbourhood Policing Inspector said: “This must be one of the most unusual locations we have discovered a cannabis grow, but it just demonstrates the length criminals will go to produce this high-value commodity.
“Once inside the drainage culvert, we found a sophisticated set up which included extensive shelving to ensure the plants were kept above water and power diverted for lighting and other equipment needed to maintain the plants.”
Lewis Judd Daventry Neighbourhood Policing Sergeant said: “This was a particularly difficult cannabis grow to dismantle due to the torrential rain and the remote location.
“The nearest road access to the tunnel was about a mile away, but thanks to the generosity and support of a local landowner who provided us with transport and trailers, we were able to close it down.”
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