Shallow Waters Along the Cavern's Edge                 -      N      - 
(-------------------------------------------------)     - <---(M)---> E
                                                        SW     S      -

You are swimming through the waters of an enormous subterranean lake. 
The curving stone wall of the cavern rises up to the west, lapped gently by
the lake's small waves.  The water is murky and dark, making it impossible
to tell what lies beneath the surface.  You doubt that anything dangerous
could survive this far underground, but you keep your eyes open just in
case.  The shoreline lies to the southwest.