A Small Island                                         -      N      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      S      -

You are standing on a small patch of land in the middle of an enormous
subterranean lake.  The ground slopes gradually above the water level, but
to the north it turns up dramatically, becoming part of a circular wall
running all the way around the island.  The resulting cylinder goes from the
floor to the ceiling of the cavern, and looks as though it may be hollow. 
There is a small hole at the bottom of the wall, just large enough to crawl
through.