From Bulbapedia, the community-driven Pokémon encyclopedia.
Southern Island is an island to the south of Hoenn. In Pokémon Ruby, Pokémon Sapphire, and Pokémon Emerald, a trainer can only get to Southern Island through use of the Template:I. To obtain the Eon Ticket in Ruby and Sapphire, a player must either connect their game to an e-Reader and scan the physical Eon Ticket and then talk to Norman in his gym, or mix records with a player who has done the above. It is currently unknown how to acquire the ticket in Emerald. The physical ticket was distributed in Nintendo Power volume 173.
To get to Southern Island, the player must go to Lilycove City and attempt to board the S.S. Tidal. The player will be taken to Southern Island. On Southern Island, the only thing of note is a rock. When the player walks up to this rock and presses A, they will be engaged in battle with either Latias or Latios (whichever is/was not found roaming Hoenn in the wild), on level 50 and holding the Template:I. A player may not return to Southern Island after battling this Pokémon.