From Bulbapedia, the community-driven Pokémon encyclopedia.
Solar Cave (Japanese: たいようの どうくつ Solar Cave) is a dungeon in Pokémon Mystery Dungeon that contains 20 floors. It is automatically accessible after the credits.
Three HMs can be found in this dungeon: Dive on 10F, Waterfall on 15F and Surf on 20F. Keys can be found on the ground in this dungeon, which are required for obtaining the HMs and getting items from similar obstacles in other dungeons. A Water Pokémon is also required to get Dive, though Whiscash will freely give out the first Dive HM. Moon Stones and Sun Stones may also be found on the ground. All pokémon in this dungeon are whole or part psychic type.