Note from Les:  Sometimes a hole in the floor is a hole, and sometimes it is a drain.  I have seen people put concrete in it thinking they are fixing it.  If you don't know what you are doing in construction, the best thing you could do is, not to touch it.

Call someone who knows.  Now a real problem in this industry is that not too many people know.  They make you believe that they know and so you trust them.  I mean why wouldn't you. The problem  is  that though an experienced handyman, who is honest,  will know many things. It is simply impossible to know everything.  If I don't know something I tell you and suggest an expert.  For instance in the case of the hole in the floor, if I was not sure, then I would call a plumber.