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The Unown Dimension is an alternate dimension that is most notable as the natural home of Unown. It has made a number of appearances in the anime. The dimension seems to have an unusually strong link with Ash's world.
The dimension was first seen in Spell of the Unown: Entei. While researching ruins with depictions of the Unown, Spencer Hale was transported into the dimension. His daughter, Molly, inherited a puzzle box found there that gave her contact with the Unown, who granted her wishes. Such was the strength of the world she created that an opening to the dimension appeared in the entrance hall of her mansion. During this time, the Unown gained powerful control over Greenfield but all contact with the dimension ended after Molly wished for things to go back to the way they had been before.
The Unown and their dimension were explained in greater detail in Address Unown!. A portal to the dimension was opened above Mt. Silver and an Unown fell out and became sick with a fever. It had little trouble returning when it was in good health.
In The Rise of Darkrai, this dimension became the battleground of Dialga and Palkia before their battle moved to Alamos Town. The presence of the Unown during their preliminary battle suggests that the Unown Dimension represented here is the same as that in Address Unown! and Spell of the Unown: Entei. Arceus and the Jewel of Life suggests that the Unown Dimension collided with the colliding dimensions of Dialga and Palkia.
The Unown Dimension also appears in PK21.