While doing some remodeling we pulled the wall off of the stud framing that backs a one piece tub/shower unit. Noticed that the hole cut into the slab for the plumbing was never closed up. Other than explaining our bad moisture and mildew problems we have noticed that the rock bed and the slab are separated by at least 6-8 inches of air. Obviously the slab couldn't have been poured this way. (over 20 yrs ago) My questions are Is this settling typical? Do we need to see how far the settling has gone under the house? Does this need to be refilled? Could the slab break apart without the rock supporting it? Lots of questions! Thanks for any advice.