The tunnelers smashed through a concrete floor which is several inches thick. They could not have done that quietly but no member of staff heard them. Almost 500 men left their cells and queued at the cell with the hole but no member of staff noticed anything. How odd.
but to suggest that masses of prisoners were involved in the plan is wide of the mark. This wasn't a question of prisoners tunnelling out, but of people tunnelling in. The prisoners had no idea about it until it happened.