From Bulbapedia, the community-driven Pokémon encyclopedia.
The Mirage Mansion is an anime-exclusive location in the Kanto region. It is the setting of The Mastermind of Mirage Pokémon.
The laboratory, which is owned by Dr. Yung, is a massive castle located on the shore of a large lake in a forest. It is filled with the dome-like generators used in Yung's Mirage System in both its interior and exterior. Ash and his friends encountered Misty and Professor Oak, who were also specially invited, in its courtyard, where they soon afterward met Yung himself. Later, they went to a battlefield to have a battle with Yung's Mirage Aggron.
Dr. Yung's true intentions, to get revenge on the Pokémon Institute for refusing his Mirage Pokémon research, quickly got underway. Posing as the Mirage Master (Japanese: ミスター・ミラージュ Mr. Mirage), he kidnapped Professor Oak and proceeded to steal data from laboratories and Pokémon Centers around the world to perfect his system. He also extracted memories from Pikachu to create an almost unbeatable Mirage Mewtwo. Ash and his friend worked together, alongside a Mirage Mew, to defeat the Mirage System.
The interior of the castle is maze-like. Its cellars lead to a seemingly endless trail of crisscrossing paths leading up the central tower. At the top of the tower is Yung's main laboratory, where the core of the Mirage System is located.
The laboratory was mostly destroyed by fire at the conclusion of the episode, after the Mirage System exploded.