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Can you see dots (i.e., the "floor") immediately
surrounding your character and moving with him/her? (You
know, same as when you're in a tunnel or a dark room in the
dungeon.) If so, what you're talking about isn't a bug, as
far as I know: the town has a day-night cycle, so to speak.
It's "night" there now, and hence dark; eventually, it'll
become light again, and so on.

Finding the stairway down is always tricky when it's dark;
it's a good idea to memorize its location while it's
still "light."

Note that I'm *not* i0lanthe--I'm just another player. So
take what I say with a grain of salt, though I'm pretty
sure I'm right.