From Bulbapedia, the community-driven Pokémon encyclopedia.
Luminous Cave (Japanese: ひかりの どうくつ Luminous Cave) is a mysterious cave which appears in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. The cave is filled with a mysterious light that speaks to whoever enters. It is used to evolve Pokémon in the game when they enter the cave.
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, and if they accept this as well, whether or not they would like to give a second item. This is used to evolve Pokémon such as Shellder and Onix, who require special items to evolve. The second item, the in-game link cable, is only used for Pokémon who require trading with a held item, to evolve. The evolutionary item must be given for the Pokémon to be able to evolve. While the cave will take any item it 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 is fully evolved, the cave will notify it and provide the reason (lacking an item, underleveled, etc.) and tell it to return later.
Luminous Cave's equivalent in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness is Luminous Spring. Unlike Luminous Cave, it does not restrict entry based on team size.