Great Canyon (Jおおいなる きょうこくGreat Canyon) is a main-story dungeon accessibly to both Red and Blue Rescue Teams. It can be accessed after the completion of Mt. Thunder and the events that unfold prior and during the dungeon exploration. It has 12 floors in total.
No weather conditions occur, and no monster houses, buried items or shops appear in any given level.
Spoiler warning! This article contains plot details about an IMPORTANT EVENT!!!!
After defeating Zapdos and saving Shiftry, the main player and their partner return back to Poke'mon Square, and are informed to go to the Hill of the Ancients, a place where a Xatu rests. This is done in order to understand the events of the recurring natural disasters and the cause of them all. The player and their partner then traverse the levels of Great Canyon, and reach a motionless Xatu. The player is then given two options, both which swing it back into action.
The partner questions Xatu about the main character's transformation from human to Poke'mon. Xatu then tells the player that the natural disasters are seemingly linked with the transformation of the main character, and simultaneously predicts the destruction of the Poke'mon world. Gengar overhears this conversation and decides to head back to the Town Square in order to spread false news.