Shallow Waters Along the Cavern's Edge                 -      N      - 
(-------------------------------------------------)     W <---(M)---> -
                                                        -      -     SE

You are swimming through the waters of an enormous subterranean lake. 
The massive stone walls of the cavern rise up to the east and the south,
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.