The Intestines Maze (肉迷宮, Niku Meikyū, Flesh Labyrinth) is a large area accessible from a number of locations, the fastest being going 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, Red City, Blood World, Guts World, Ecstasy World, Soldier Row, Netherworld, Hand Fields, and Flesh Paths World. Directions are included in the map below.
Nexus → Geometry World → Intestines Maze