It is unlocked when Deputy Magnemite tells the player the whereabouts of Drowzee after Azurill has fallen under a nightmare he cannot wake up from, which Drowzee can reportedly enter. On the first visit, the player finds Drowzee at Mt. Travail Peak. The dungeon itself consists of 20 floors total, and tends to vary in weather conditions.
This location is only available via download. This dungeon has no boss.
Moves will level up faster here, and both manuals and training seeds can be found here as well. There are also special tiles that increase your move level-up rate.
The artwork at the beginning of the dungeon is the same as that of Mt. Thunder in Pokémon Mystery Dungeon Blue & Red Rescue Team, just with a different color scheme.