Cramming Nerath/PoL into Mystara
by Giampaolo AgostaSpeaking about the Hollow World, there are several areas that more or less fit the map of Nerath -- the Sea of Yr, the Sound of Yith, the Bay of Adoth and the Bay of Lokam are all more or less circular seas mostly surrounded by land.
The Hollow World is also obviously smaller than the Outer World, which helps with the different climates in a small space.
My favorite solution here would probably be to flip horizontally and vertically the map, so the Iron Circle matches the northern coast of the Bay of Adoth (with Nithia and Pflarr's Wall as the "Impassable Territory"!), Nerath fits the southern coast, and Karkoth and Vailin occupy the southern reaches of the continent of Suridal. You would need to adjust things a bit, but not too much (mainly, Vailin is rotated towards the Bay of Lokam to leave space fo the Sea of Rax).
Here is a rough mashup of the two maps
