A seemingly fruitless marijuana grow-op bust in Surrey, B.C., took a surprising turn when investigators realized a basement bookshelf was actually a swinging trap door.

Surrey RCMP's Marijuana Enforcement Team was dispatched to a house in the Tynehead-Fleetwood area Thursday morning, on a tip of a possibly significant pot operation.

But no trace of a grow-op was found, until investigators noticed a large bookshelf in the basement that appeared out of place.

What they discovered was a trap door - a cabinet on a hinge, which police swung open to discover the entrance to a walk-in safe.

Inside the safe police found a fully-operational grow-op, where pot plants were harvested from the cloning stage to mature plants, including a drying area to street-ready the product.

In total, 276 plants and 25 pounds of ready-to-sell marijuana were seized, with an estimated value of $300,000.

A 46-year-old man and 31-year-old woman were arrested at the home, and are being held in custody as the investigation continues.