A Perfectly Smooth Corridor                            -      -     NE 
(-------------------------------------------------)     - <---(M)---> -
                                                        -      S      -

This circular corridor isn't perfectly circular, or at least not
inbetween its segments.  An edge exists here, paradoxically, a circular
edge, at which it intersects a similarly smooth circular corridor, off to
the northeast.  As it travels northward, this passage declines a degree or
two, barely perceivable, but still enough to add up, over time.