Banankoki is the name of the original pangea continent, and also effectively the name for the world itself
It is technically a cake-shaped planet I guess.
At the heart of Banankoki is the underdeep, a primordial sea.
On top of that is a layer of a vast underground world.
And on top of that, the overworld, which to its inhabitants appears like a wide disc with a few large island continents in the center.
And on the other side of the underdeep, a mirror image: the overunder and the otherworld.
Water is pushed up from the underdeep to the surface, where it cascades down the sides of the cake and eventually rejoins the ocean.