I love Hollow Earth theories. There's some truly insane worldbuilding that's gone into describing what's inside there. Sometimes it becomes this bizarre multidimensional thing with layers of magical effects the deeper you go, and some sort of a primordial chaos forest or something at the bottom. edit: The primordial chaos forest is also infinite, which is important

I tried to find my favorite long-winded text on the topic which goes really deep into the weeds with histories of various alien civilizations and shit, but this is the best I could quickly find.