BARRIE -- A marijuana growing operation at the former Molson brewery in the city -- thought to be the largest ever discovered in the province -- is twice as big as police originally thought. And a second huge marijuana grow house was discovered just north of the city yesterday when police raided a commercial building in Oro Medonte.

"To give you some perspective, think about this," Ontario Provincial Police Supt. Bill Crate said yesterday. "An average grow house in a residential area can produce about 500 plants.

"At the one in Oro Medonte, they found thousands and thousands of plants and that was small compared to the brewery," he said.

The former brewery wouldn't normally be considered a good site for a secret pot-growing operation.

Millions of drivers commuting through southern Ontario over the years on Highway 400 could see into the landmark which is just metres from the road and features huge windows.

The six-lane highway is the main route to cottage and ski country from Toronto and is jammed almost every weekend.

Eleven people -- including one found hiding in the former retail store near the brewery yesterday -- have been arrested.

The raids were carried out as part of a probe into illegal grow-house operations, which police say have increased dramatically in recent years.

Crate, who declined to say how police learned of this particular operation, toured the brewery site yesterday and said he was amazed by what he saw.

"There was everything from seedlings to mature plants to plants that they were just readying for sale," he said.

"It's really pretty amazing. I think everyone was surprised by the size of it all. We thought it was huge yesterday (Saturday), and today it's twice as big."

Police described the operation as a "sophisticated, well-organized" setup with an elaborate and monitored growing system.

"There are lots of offices, working spaces and nooks and crannies everywhere," Crate said, noting plants were discovered growing in several locations all over the building, including inside the beer vats, some of which can be seen from Highway 400.

Police also discovered living accommodations inside the plant.

"If you think of this as a factory with people running the operation 24 hours a day, it fits that there would be sleeping accommodations," Crate said.

The former brewery, which Molson closed in 2000, now houses about half a dozen companies that have leased space from Rosten Investments.

Rosten bought the property with plans to open a beverage bottling company. Those plans have never been realized.

More than 100 officers from the Huronia combined forces drug unit raided the Barrie landmark Saturday morning.

"We first have to ensure the safety of our officers," Crate said. "There was all kinds of hydro being misused and we have to look for booby traps that are often set up in this kind of operation."

No traps were discovered and the police search and seizure investigation began yesterday afternoon. The investigation is expected to last a few days.