One day last summer, residents of rural Norfolk began to complain about a strange and pungent smell wafting across the Fens just south of King's Lynn.
Soon, officers were knocking on the door of a massive greenhouse complex covering roughly 18 hectares of farmland (the size of 23 football pitches) next to the River Wissey, on the outskirts of Downham Market.
The facility, owned by British Sugar, had originally been built to cultivate tomatoes. But in recent years, the site had quietly undergone an intriguing transformation.
Today, it happens to be the location of Britain's only legal cannabis farm.
Here, thanks to a special licence quietly granted by the Home Office, a huge crop of marijuana was being cultivated on behalf of a British drug company called GW Pharmaceuticals.
Was this the source of the noxious smell? Opinion was, and still is, divided. Some locals insisted it was. But while Fenland District Council told reporters that the smell did indeed appear to be that of cannabis, they found no evidence the British Sugar plant was responsible.
A month-long investigation was unable to reach any firm conclusion, although it did suggest that nearby waste disposal plants might also be to blame.
Either way, the affair highlighted an intriguing, but little-known fact: that this vast horticultural unit, down a secluded country lane and largely shielded from view by hedgerows, is almost singlehandedly responsible for Britain's status as the world's largest producer of medical cannabis.