Fury Below (1936)

Jim Cole, heir to a mining operation, takes over the mine, which is suffering from unexplained low production, and is facing a strike by the miners. Jim will soon loose the mine if production doesn't increase and the sabotage continues. Mary Norsen, office-secretary, accidentally learns of a plot by Fred Johnson to wreck the mine and force Cole to sell his coal-mine. She and her brother, Joe, join Cole in his fight to stop Johnson.