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Fire Red & Leaf Green: Mystery Islands
In Fire Red and Leaf Green, there are Mystery Islands you can travel to. The use of the islands isn't known yet, except that youi can meet Bill and a new character named Nishiki. They could have Gyms, or could have Battle Towers. They are named Island 1, Island 2, etc.; but there isn't an exact number as to how many there are.

Traveling by boat to each island.
This is the island where you meet Bill, the creator of the Pokemon Storage system.
This is Mystery Island 1, possibly where you meet Nishiki.