From Bulbapedia, the community-driven Pokémon encyclopedia.
Seafloor Cave (Japanese: かいていどうくつ Seafloor Cave) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It is automatically obtained upon recruiting Kyogre. It is in the Southern Sea area, and has a capacity of one Pokémon.
It is a crystal cave deep underwater. There are markings on the floor similar to the ones on Kyogre, which change from blue to red.
It is said that a mythological Pokémon, spoken of through the ages, lives inside this unexplored cavern.