A Perfectly Smooth Corridor                            -      N      - 
(-------------------------------------------------)     - <---(M)---> -
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This circular corridor is very strange, too perfect to be made by man,
but also too perfect to be made by nature.  This corridor disappears
northward, stretching out into the darkness.  Its inside surface is a
perfect surface, about three yards in diameter.  There isn't a scratch or
imperfection to be seen, and there is no indication to who or what made this
bizarre tunnel.  The southern edge is perfectly flat, intersecting its
ceiling, walls and floor all at the perpendicular.  No such edge exists to
the north, although if you were to continue on in that direction, you could
probably find one.