The Black Wall of Moldrath

The Wall runs the full width of the north, coast to coast, about three thousand feet high and unbroken. It is black, cold to the touch in summer, and faintly warm in winter. It hums on still nights. Nobody built it, as far as the histories know, and nobody who has gone over or through it has come back to say what is on the other side. The Guild of Lines forbids mapping past it. Not because it can't be done, they say, but because the maps never agree. That's the question the whole series stands on.