London police have just discovered a massive cannabis grow-operation hidden deep in the basement of a former Victorian theater.

Police discovered the black market weed operation in early October after being notified of an attempted break-in at a residential house in Deptford, Southeast London. While investigating the house, cops discovered that a ground-level bathroom had a hollow floor. Upon closer investigation, the hollow floor turned out to be a trap door leading to a 70-foot-long underground passage.

In a massive room at the end of the passage, cops discovered a “large number of cannabis plants along with the equipment used in the cultivation of cannabis,” the News Shopper reports. Police estimated the value of the seized weed plants and cultivation equipment at over $51 million. Authorities believe that the pot farm has been operating for as long as a decade, and may have been bringing its owners more than $5 million a year.

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The grow-op was set up in the basement of what was once a 750-seat Broadway Theater, built in 1897. This theater was demolished in 1963 and replaced with a number of shops and residences, but the basements and foundations of the theater remained intact. The crew responsible for the grow op reportedly dug a tunnel from the bathroom of one of these houses, covering it with a hidden trap door.

The tunnel stretched underneath a private parking garage and eventually reached the large, abandoned basement of the theater, which was repurposed into a successful black market weed farm. The growers dug two separate air shafts into the basement, to be used for ventilation and possible escape routes. Police also believe that they tapped into the city’s main power lines to provide the electricity for grow lights and heat.

“To think this has been happening next to us for ten years without any of us noticing is certainly surprising,” a businessman who lived next door to the grow-op told the Daily Mail. “I was shocked. My wife and I sometimes heard what seemed like drilling. It was never for very long but it vibrated the walls upstairs. We had no idea what it was, we just thought it was some building work nearby — it’s a busy road.”

A police spokesperson said that “three men, aged 28, 45, and 47, and a 36-year-old woman have been arrested on suspicion of the cultivation of cannabis. They have all been released under investigation.”

Around the same time that the cops uncovered this secret theater grow, Italian police discovered a similar — but completely unrelated — hidden weed farm in Sardinia. Access to this secret 5,000-square-foot grow room, which held over 500 plants, was also hidden behind a small trap door in a residential bathroom.