From Bulbapedia, the community-driven Pokémon encyclopedia.
Luminous Cave (Japanese: ひかりの どうくつ Cave of Light) is a cave in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team where the player can evolve Pokémon. The cave is filled with a mysterious light that speaks to whoever that enters.
After the player defeats Rayquaza at the Sky Tower Summit and convinces Rayquaza to destroy the meteor headed for Earth, a part of the meteor hits the round stone in the middle of Whiscash Pond, creating the Luminous Cave. It is discovered by Snubbull, who enters and evolves into Granbull.
The cave's small size makes it so that only one Pokémon can enter. Upon entering, the player will be asked if they would like to evolve. If they accept, the cave will then ask if they would like to give an item for evolution, if no the Pokémon will evolve if possible or not if they are not eligible. If choosing yes and an eligible evolutionary item, the Pokémon will evolve or the cave will ask for an additionary item if the Pokémon can evolve using an other item in conjunction.
While the cave will take any item that is offered regardless of whether or not it is required for the player's evolution, it will return all items offered if the player does not meet evolutionary requirements.
If the player's lead Pokémon does not meet its evolutionary requirements, or cannot evolve further, the cave will notify it, provide the reason, and tell it to return later.
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