Here's a few more maps from the Marvel Universe which could probably be put to game use--or at least provide some inspiration.
I've been chronicling the Warlord's adventures in the lost world of Skartaris, but Earth isn't the only hollow world out there in comics. Titan, the moon of Saturn, is hollow in the Marvel Universe and the home, according to Jim Starlin, of an offshoot race of Titans (later retconned to be Eternals).
Returning to more earthly locales, how about a sandbox set in a tiny kingdom ruled by a tyrannical wizard who hides his facial deformity behind an iron mask? Well, welcome to Latveria and its capital of Doomstadt:
Once you're there, might as well explore Doom's castle, so here's an overhead schematic of it. Note the "dungeons (sub-basements)":
12 minutes ago