Atlantis was an ancient city on the planet Earth. For unknown reasons it was lost for much of human history, fading into legend. However, there was a secret passage to Atlantis that could be reached through the Sewers. Xaq took this passage to discover the city. (Series 1: "Atlantis Quest")
Additionally, Indiana Jones went on his own quest for Atlantis at some point. ("Indiana Jones and the Fate of Atlantis")
- Some version of the Indiana Jones story is canon, as there was a PCG adaptation, though it's not certain if it takes place in the PCG Universe or not.