Also with the covered underbelly, the manufacturers take less time in cutting penetrations through the floor because the underbelly will cover the large holes. Without the underbelly, holes for pipes are the size of the pipes. With an underbelly, holes are cut much larger than the pipes and are great areas for mice to travel at ease up from the underbelly. It looks like most holes are cut randomly with saber saws and are just about any shape imaginable. Also, no need to clean up any sawdust or debris because it will all be out of sight.