Is it possible that the third heaven is like in another dimension, not part of the natural space-time continuum that we live in?
Also, is it possible that the bottomless pit may also be in another dimension, and not necessarily part of the natural earth? If so, then it could easily be bottomless. And perhaps in that sense, the further down the pit, the more horrible the creatures are, the more terrifying it becomes, the darker it becomes, and the more painful it is?