|This article is about the Underswap fan game by Team Switched|
As the game is in development, this information may become inaccurate over time.
Coldsea is a place covered mostly by snow on what seems to be solid ice. Several of the puzzles that are here are used technology at the CORE, as well as the low temperature it has.
After defeating Alphys, the protagonist enters Coldsea. Three sections can be found, one that directs to W.D. Gaster, one to Undyne's laboratory and another blocked by Bratty and Catty, which leads to the elevator that accesses all coldsea's regions. After entering the laboratory, a chat with Undyne, and a challenge by Napstaton for the first time, Undyne modifies the phone of the actor to add extras, among some other 'applications' that will be used in combat against Napstaton along Coldsea two storage boxes. On their journey, the protagonist must deal with Bratty and Catty, solve several puzzles and survive a bout against Grillby. At the end of Coldsea, the protagonist reaches NTT Resort, where you can talk with Papyrus, the RG 01 and RG 02 or within the NTT Resort shop, or simply to move forward toward the CORE.
Once the player has successfully defeated Alphys the Alpha, the protagonist can reach Coldsea. Inside the lab you will find Napstaton, who will not take no special action since Undyne is hidden with the rest of the monsters, refusing to help the protagonist. Several safety courses can be found along Coldsea, two completely blocking the entrance to the second floor.