The Stone World (404洞窟, 404 Dōkutsu, 404 Cave) is a currently unfinished area in Yume 2kki, accessible only through a part of Geometry World. It currently has no exit, forcing you to either wake up or use the Eyeball Bomb Effect to leave.
Features
This unfinished area contains very little as of .94f, 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.
A solitary vending machine is also in the area. Drinking a can from this machine causes the sreen to turn green, and Urotsuki to pass out. After a while, she will wake up in the same spot.
Also of note is a ladder, hidden further inside the world. This ladder will take you to Monochrome GB World. There is also another flashing portal to the Digital Hand Hub in this world.
Connections
The Hand Hub (indirectly, one way)
Digital Hand Hub (One Way)