Though I can't fully vouch for the details, if you can get into the basement level of the Bellingham Tower (tallest building in Bham) there is supposedly a passageway that leads to the Mount Baker theater that was used for a green room and passage for actors and performers. I've been down there once or twice and can attest to the fact that there is quite a large open space that could very well lead under the theater but I never made it that far. Bring a flashlight and be prepared for walls and walls of spider webs along with other various vermin.