Northern River Bank                                    -      N      - 
(-------------------------------------------------)     - <---(M)---> E
                                                        -      S      -

You have crossed the underground river, and this is it's northern bank. 
The sounds of the river echo all around you, and you can barely hear
yourself think.  To your north, a greenish glow emanates from the very walls
of the cave; to your south, you see the river, and beyond that, the entrance
to this massive cavern.