My downstairs neighbor's floor is buckling upward in her bedroom. She lives on the ground floor, her bedroom carpeted with old wood floors underneath (building was built in 1957). It's a seven unit building in southern California, with a crawl space under the foundation. Plumber sent by landlord said he detected no leaks underneath the building though we're not so sure. Her room also smells musty. Any one have any ideas what could be causing this?

we have had this problem to it was dew to condensation which is dew to not having enough vents in foundation for air flow. hope this helps

Try a dehumidifier. It sounds like to much moisture in the area. Basements areas are bad for that. The dehumidifier will help pull the moisture out. Hope this helps.