I purchased a new condo build in Iowa in 2009, just behind the basement thb/shower there is an open space. It shows the plumbing going into the gravel under the tub. As seen from the unfinished part of the basement there is an open area appx 24" and as far back as I can see where there is just dirt and gravel. Does it sound like it is OK to leave this open space or should it have been filed with concrete to secure and fill up the opening. I would think that any creature or insect that can tunnel could enter the basement very easily. Any Ideas on the codes associated with basement floors. And what about Radon?