Undersail is an AU that takes place underwater in the Ebott Triangle. This AU was made as a design aesthetic and had no canon. There is also no plans for canon, either.
- Frisk is a diver who gets an air bubble from Toriel.
- Sans and Papyrus are both pirates.
- Flowey is an anemone.
- Toriel is a capricorn.
- Undyne is a mermaid.
- Alphys is an axolotl.
- Napstablook is a jellyfish ghost.
- Mettaton is a bathyscaphe.