Sharply Sloping Tunnel                                 -      N      - 
(-------------------------------------------------)     - <---(M)---> -
                                                        -      S      -

Following this narrow passageway is rather difficult, since the floor is
almost totally smooth and is very steep.  You brace yourself aginst both
walls to avoid slipping, and attempt to continue safely on your way.  The
tunnel leads down to the north and up to the south.