3D Underworld is an area accessible through the cone house with the faucet in Marshmallow Shallows.
This area is a distinctive variation of FC Caverns that connects to several areas. Though it shares the identical platforming movements of FC Caverns, 3D Underworld's structure has more of the same isometric, three-dimensional style of Oblique Hell, and also features the obstacle of red walls which periodically slide open and shut. The Fairy effect is recommended to navigate and reach the other connections in this area.
From Marshmallow Shallows, the player will fall through a couple of screens, some of which have platforms leading to other areas. After falling to the bottom and passing through a moving platform, you can find a door with a dark blue, scrolling parallax similar to that of Bubble World's, though entering it will instead lead you to the Glitch Nexus, which is a vertically looping subarea of 3D Underworld. From here, interacting with the 3D sculpture-like NPC n the middle brings you to Oblique Hell. Heading left in the Glitch Nexus leads back into Marshmallow Shallows in the uterus room as a one-way trip. Heading right will send you to Oblique Hell's First-Person Maze.
By going slightly north of where you had initially landed, taking the west platform will lead to a short hallway with moving blocks, ending in a round portal with a colorful exit, leading to Kaleidoscope World. You can also choose to go north from here and reach another portal with a pale light shining over a black ocean, leading to Streetlight Docks.
- The Nexus → Marijuana Goddess World → Color Cubes World → FC Caverns C → Home Within Nowhere → Pure White Lands → Lamplit Stones → Adabana Gardens/Field of Cosmos → Bubble World → Red Sewers → Marshmallow Shallows → 3D Underworld
If you have visited Red Sewers at least once and have the Cake effect:
- The Nexus → Urotsuki's Dream Apartments → Dressing Room → Fabric World → Guts World → Red Sewers → Marshmallow Shallows → 3D Underworld
- The Nexus → Library → Broken Faces Area → Dream Park → Guts World → Red Sewers → Marshmallow Shallows → 3D Underworld
- Viewing this map in RPGMaker will reveal that this map's layout is identical to Oblique Hell, though flipped upside down.
- Additionally, this map features collectable coins when viewed in RPGmaker, though as of current, they are inaccessible. These same coins can also be found when viewing FC Caverns C.