I'm looking for examples and inspiration for mapping a region and encounters that take place on the elemental plane of earth(EPoE) (D&D 5E). Given published descriptions of the EPoE, I'm envisioning an underground place(s) with various large pockets that the players can explore.

(For background, last night my players went through the portal to the EPoE in the Princes of the Apocolypse adventure. So now they are out of the bounds of the adventure and I need to come up with content, which is fine, just means I have things to do!)

I guess what I'm looking for are example of regional and local/encounter maps of a subterranean nature that have an elemental feel for them. I'd also like to incorporate various geological anomalies for interest and creativity.

Thanks in Advance.