Shallow Waters Along the Cavern's Edge                 -      -      - 
(-------------------------------------------------)     - <---(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 north, lapped gently by
the lake's small waves.  You are unable to see the shoreline through the
darkness, and it is also impossible to tell what lies beneath the surface of
the water.  You doubt that anything dangerous could survive this far
underground, but you keep your eyes open just in case.