The Sign World is a colorful world accessible from several different areas, such as the Bug Maze.
Features
The most prominent feature of Sign World is the pyramid, which plays a significant role both in obtaining the Gyroid effect and in reaching the Power Plant. The pyramid can be reached by traveling northwest from the Bug Maze entrance.
By traveling south from the Bug Maze entrance, then heading east after reaching the second sign, you'll find a portal to a small room with a moving heart. Interacting with the heart takes you to Heart Land (NOT Heart World).
A sign that drops Urotsuki into the Teleporter Maze can be found by traveling directly north from the portal to the heart room. It's easy to spot, as it's blue and white versus the grey and yellow of the other signs.
If you travel north from the Bug Maze entrance, then change to west after seeing 5 signs, you'll find a red smiling face next to a sign. Interacting with the sign will result in Urotsuki being teleported into an enclosed area filled with signs with a faster version of Sign World's music playing. Access to the menu is disabled here. After a moment, all of the signs and Urotsuki will fade to white, and Urotsuki will appear in front of a crescent-shaped fragment of a smiling face.
Connections
Gyroid effect area
Heart Land (indirectly)