A Deeper Part of the Lake                              -      N      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      S      -

You are swimming through the waters of an enormous subterranean lake. 
The shoreline is not visible through the darkness, and it is also impossible
to tell what lies beneath the surface of the water.  However, this section
of the lake seems deep enough for you to dive underwater for further
exploration.  You doubt that anything dangerous could survive this far
underground, but you keep your eyes open just in case.