Peril on the Purple Planet is an excellent mini-campaign from Goodman Games. I got the boxed set as part of the Kickstarter, plus all the add-on modules (either in print or pdf).
I could run it pretty much as-is, as a distant world accessed via a portal or magical mishap. However, I think that it could also be a good fit for Underborea.
For my Underborea, the entire plateau described in Purple Planet could instead be a vast subterranean cavern, illuminated dimly by the phosphorescent purple fungus that grows upon the cavern walls and ceiling. The plateau itself is about 300 miles by 200 miles, so it fits nicely beneath the Spiral Mountain Array.
I've posted some conversion notes and additional thoughts on my gameblog.