From Bulbapedia, the community-driven Pokémon encyclopedia.
Cerise Island (Japanese: スオウ島 Suō Island) is a location in the Pokémon Adventures manga. It was located somewhere to the west of Cinnabar Island. The island is unmapped and almost unknown. There is also a dormant volcano inside of it. It is called a "gigantic badge energy amplifier" by Lance as with seven of the Badges located around the perimeter of the island under its rock spires and the Earth Badge in the center of the island over the volcano, it performs the same function as the device used by Team Rocket, but on a much grander scale.
It was the headquarters of the Elite Four during the Yellow chapter, whose plans were to decimate the entire human race save for themselves and their allied Trainers. They felt that humans were destroying Pokémon and only by killing all humans could the Pokémon be saved.
Cerise Island appears in the Pokémon Adventures manga, volumes 4-7, where Yellow, Bill, Blue, Green, Red, Koga, Sabrina and Lt. Surge battle against Lance, Agatha, Bruno and Lorelei in order to save the human race. Lugia also makes an appearance above Cerise Island.
Due to Chuang Yi separately translating the manga, the area is also called Salt Island in English.