The Intestines Maze is a large area accessible from a number of locations, the fastest being through blue Smokin' HAL's mouth in Geometry World.
The Intestines Maze is a huge maze with pulsating pink walls, which houses a variety of portals between far-off areas. A mixture of eyeballs and flesh can be found scattered throughout the area, and it is also home to a number of Chasers. This area seems to be the equivalent of Hell from Yume Nikki.
You can enter the maze from a number of locations, but the fastest are the portals from the Hospital and Geometry World.
Through its variety of portals, the maze can serve as a travel hub between the Hospital, the Red City, Blood World, Guts World, Ecstasy World, Soldier Row, the Netherworld, the Hand Fields, Flesh Paths World, and the DNA Room. Directions are included in the map below.
Nexus → Geometry World → Intestines Maze