From Bulbapedia, the community-driven Pokémon encyclopedia.
Entrance of the Pokémon Mansion.
The Pokémon Mansion is a decrepit, burned-down mansion on Cinnabar Island. It got its name because a famous Pokémon Researcher once lived there.
To access the Cinnabar Island Gym in Generation I and Generation III, the main character has to enter the mansion and find the Template:I to unlock the door of the Gym.
The Pokémon Mansion has four different floors. Doors can be unlocked in the Mansion by pressing switches hidden in Rhydon statues.
Items found in the Generation I version of the mansion include an Escape Rope, a Carbos, a Calcium, an Iron, a Max Potion, a Rare Candy, TM 22 Solarbeam, a Full Restore, TM 14 Blizzard, and of course the Secret Key that unlocks Blaine's Gym.
Amongst the rubble and wreckage is information that the scientists who once worked there obtained a Mew and impregnated it with the genetically altered Mewtwo, who destroyed the Mansion in its escape.
Pokémon that Appear in the Pokémon Mansion