From Bulbapedia, the community-driven Pokémon encyclopedia.
Cryptic Cave (Japanese: なぞのどうくつ Mystery Cave) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It can be bought for 6,500 Poké from Wigglytuff in Pokémon Square. It is in the Cave area, and has the capacity of only one Pokémon.
It is a cavern, with sharp rocks surrounding a central platform. There are shining specks in the distance.
This very mysterious spot, said to be deep inside a dungeon, is the perfect place to store energy without expending any effort.