Cinnabar Island is an island located in the southwestern part of the map. It is home to the Pokémon Mansion; the Pokémon Lab; and the Cinnabar Gym, where you can defeat Gym Leader Blaine to receive the Volcano Badge, although, you won't be able to enter the Cinnabar Gym, until you get the Secret Key from Pokémon Mansion.
Pokémon Mansion
The Pokémon Mansion is a decrepit, burned-down mansion. It got its name because a famous Pokémon Researcher once lived there. 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.
To access the Cinnabar Gym, you have to enter the mansion and find the Secret Key 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 Pokémon statues.
Pokémon Lab
The Pokémon Lab is a laboratory founded by Dr. Fuji.
In the first room of the Pokémon Lab, there are two people who will offer to trade Pokémon. The old man will offer his Electrode for a Raichu, and the girl will want to trade her Tangela for a Venonat.
In the second room, there is a man who will tutor Metronome to one of your Pokémon, what Metronome does is that it randomly selects a move and fully executes the attack, including moves affected by Disable.
In the third room, there is a scientist who will revive the fossils the player obtained at Mt. Moon and the museum in Pewter City, however you must walk out of the building before returning to claim the fossil Pokémon. There is also a third in-game trade: Seel for a Ponyta.