I can't remember who actually was the writer for the story where Tryoc leaves the Legion & Marzaal (?) goes to another dimension but it reveals that the island was founded by survivors of a slave ship from the 1700s or so. The island was like Brigadoon appearing in our universe only so long before dissapearing. It was a nice way of explaining how this society could exist in a future where the human race was getting along fine with aliens.
