If the player has the Eon Ticket, they can travel to Southern Island. In Pokémon Ruby, Sapphire, and Emerald, this is the only way to access Southern Island.
In Pokémon Ruby, Sapphire, and Emerald, the player must first receive the Eon Ticket from Norman. With the Eon Ticket in the Bag, the player must then attempt to board the S.S. Tidal from the Lilycove City Harbor. A Sailor will replace the usual guide, taking the player to Southern Island. In the center of the island's forest there is a mysterious sphere; upon interacting with it, the player will be attacked by a wild Latias or Latios (whichever did not roam Hoenn).
In Pokémon Ruby and Sapphire, the player cannot return to Southern Island after battling this Pokémon even if defeated.