The world is not a bubble.
The passage between two worlds is not a culvert connecting bubbles, with entrances and exits shaped like cavities.
The entrance to the Black Prison in the sky above Buji Island is a black moon.
The shadow of the Wizard World falling upon the Black Prison World is a long white rainbow—'the World's Door' whose color reflects the characteristics of each world, and whose appearance depends on the shape formed when different dimensions intersect.
It is roughly akin to a stick in the three-dimensional world which may appear as a circular surface in a two-dimensional world; and the shadow of the two-dimensional world cast on that three-dimensional stick might be nothing more than a long, narrow mark.
