Illuminated Tunnel                                     -      N      - 
(-------------------------------------------------)     - <---(M)---> -
                                                        -      S      -

Once through the gap to the north, the tunnel is so narrow you are unable
to turn yourself around.  Because of this, the only way you can go is
onwards.  Shards of light penetrate from holes in the ceiling, nearly
blinding you now that you have spent so long in darkness.  Maybe these rays
of light are from the sun.