So hold on, whuh... I don't quite get this whole wrap-around thing. Perhaps the strangely large number of z-levels, the cave-in message on embark and the huge spire of adamantine give us a clue as to what's happened here... *ponders* perhaps the adamantine was so low beneath the ground that it wrapped round, went to the top of the sky, and then fell down, for our dwarves to mine!!! (or not mine, as the case seems to be...
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Or is that the wholre wrap-around theory anyway?
My theory is an expansion of this.
I. Adamantine tubes are actually much taller than observed, say 2000Z, due to Slade only being the vertical equivalent of a Large Cluster.
II. During WorldGen, an occurrence came from a very specific set of advanced settings, and an almost infinitely improbable generation of one location on a tube caused a cave in event, similar to how a poster described a cave-in triggered by a pool on the same spot as a volcano. This in turn, leads to the adamantine tube as a whole "construction" by itself trying to descend. Since there were no unblocked squares in the path to Z-∞, it began at the top, Z+∞. It detects open squares below, and goes to them. This eventually leads to a spire of adamantine at Z=0.