RIVERSIDE (CBSLA)A massive illegal marijuana grow spanning an entire city block was discovered by authorities in Riverside after neighbors noticed a strong smell.

About 40,000 plants were found inside of a nursery greenhouse, with the potential to produce 20,000 pounds of pot, an estimated street value of $20 million.

The hidden plants were conveniently concealed behind a nursery and nestled in between the orange groves in Riverside’s scenic greenbelt neighborhood.

Neighbor Loretta Lewis said, “I couldn’t believe it,” and explained how her children asked her about the smell.

A massive illegal marijuana grow found by SWAT in Riverside (credit: CBSLA)

She believed the nursery was growing Chinese herbs, but then her kids started asking her about a strong smell saying, “‘Mom can you smell that smell? It’s pretty rank.'”

Neighbors began calling police complaining of an overpowering odor emanating from the recently ripened buds.

“Our narcotics unit came out here and started their investigation which included surveillance,” said Ryan Railsback of Riverside Police Department.

When police had an idea of what was going on, they had a swat team execute a search warrant. Detectives used chainsaws to destroy the young plants and seized three and a half tons of mature plants.

The scale of the operation was unlike any Riverside Police had ever seen.

“Our narcotics unit believes this is the largest one that we’ve ever uncovered within our city limits,” Railsback told CBSLA.

By the time Swat arrived, police say the growers were long gone but they did find several cars, a tractor, a forklift, and abandoned animals.

Neighbors can’t believe the dangerous conditions that were right under their noses including makeshift dwellings, unsafe wiring, and loads of pesticides and fertilizer.

Riverside Police believe that at some point in time the property was a legitimate nursery, but they don’t know how long those plants have been there.