A Crystal Corridor                                     -      N      - 
(-------------------------------------------------)     - <---(M)---> -
                                                        -      -      -

The tunnel has transformed into this large corridor hewn out of a huge
formation of quartz crystal.  The walls are smooth, and the corners where
the ceiling and floor meet the walls are precise 90 degree angles.  A light
glow penetrates the corridor from the quartz, making artificial light
unnecessary.  A staircase leads up towards the surface and the corridor
continues to the north.