TMZ reports that Dr. Dre is allegedly planning to build an underground expansion to his mansion in Los Angeles. City documents show that Dre plans to dig up a large portion of the estate, which is almost the size of his home. He’s building some sort of underground room with several lightwells. It’s unknown what the room will be used for. He obtained permits to build retaining walls around his home and a guard house.

It’s unknown how much the project will cost, but it could end up costing millions of dollars when accounting for excavation, additions and construction. Dre applied for the permits several years ago. They were only granted this month.

The Brentwood home is on a four-acre property. It includes an infinity pool with ocean and canyon views, a poolside cabana, a gym, an outdoor kitchen and living area, seven fireplaces and a moat. He bought it in 2014 from Tom Brady and Gisele for $40 million. He recently bought another home in Calabasas for $4.9 million.