From Bulbapedia, the community-driven Pokémon encyclopedia.
Mt. Cleft is a friend area is Pokémon Mystery Dungeon that can be obtained by either buying it from Wigglytuff for 5000 Poké or if Charmander is chosen as the player character or the player's partner. It is in the Mountain Range area, and has a capacity of nine Pokémon.
Mt. Cleft is on a pillar with a small bridge.
This area, deep among the craggy mountains, is a gathering spot for physically tough Pokémon.