High crime: Secret tunnel to a massive underground cannabis farm found under former Victorian theatre in London

British police stumbled upon a huge subterranean drug farm after discovering a secret tunnel in the bathroom of a house in southeast London. The illegal operation is believed to have netted tens of millions of dollars.

Authorities chanced upon the tunnel after responding to an attempted break-in at an unoccupied home in Deptford, the Daily Mail reported. Upon inspection, police found a trap door in the building’s bathroom, which served as an entrance to a 70-foot (21 meters) tunnel. The passage led to the basement of a former Victorian theatre, which housed a large cannabis farm.

A Metropolitan Police spokesman said that a "large number of cannabis plants along with equipment used in the cultivation of cannabis" were discovered in the underground farm. Three men and one woman have been arrested in connection with the operation.

Detectives believe the drug factory may have been operational for about a decade, raking in more than £4million ($5.1 million) each year.

The operation was reportedly highly sophisticated, with the criminal gang even tapping into underground cables in order to heat their marijuana plants.The size of the farm is unusual – especially for such a high-traffic area like southeast London. However, British cops have uncovered similar plots in the past. Earlier this year, an underground bunker filled with lights, fans and heaters was discovered in Wolverhampton. In 2017, three men were caught growing weed in an abandoned nuclear bunker in Wiltshire.

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