There's something strange at the Earth's core.
Needs a Better Title, Needs a Better Description, Needs More Examples. Rolling Updates.
We all know the composition of the Earth. There's the crust of solid rock, a mantle of magma, an outer core of molten iron, and an inner core of crystallized iron. Fiction looked at that and said "Boring!" In fiction, the core of the Earth may have any number of weird and wonderful surprises, ranging from lost cavern gardens to ancient superweapons. Subtrope of World Shapes.
Examples:Anime and Manga