The Stone World (404洞窟, 404 Dōkutsu, 404 Cave) is an area accessible through Geometry World, by doing the Eye-Chase Puzzle, choosing the correct paths, and interacting with the red thing at the bottom of the path.
This area contains very little, and what is there seems to be quite glitchy. Parts of the floor animate, and it is possible to get inside the walls at some points. This may be intentional, based on the "404" in its name.
A solitary Vending Machine is also in the area, that sells expired drinks. Drinking a can from this machine causes the screen to turn green, and Urotsuki to pass out. After a while, she will wake up in the same spot.
A ladder, found further inside the world, will take you to Monochrome GB World.
There is also a flashing portal to the Digital Hand Hub in this world.
Nexus → Geometry World → Stone World