I know that "underground palace" can be a translation of "地宮", but I'm not too sure on the "correct" translation.
Seeing some pictures on baike, "underground palace" doesn't really seem to fit what the pictures depict.
From what I see, I think that something like "underground maze" would suit it better, stemming from "迷宮". However, with the English translation of "underground palace", should the "palace" then not be taken literally as a "palace"? I'm a bit confused since from the pictures, "地宮" seems to be more like an "underground tomb" than a "palace".