The Old Sea Map is a Key Item in the Hoenn region games which allows the player to access a special island known as Faraway Island. This item was introduced during Generation III and with the map it allows you to capture the Legendary Pokémon Mew. It is one of three Hoenn based event items, alongside the Mystic Ticket and the Eon Ticket.
This event is accessed by bringing the Old Sea Map to the port at Lilycove City, where an elderly woman will approach the player. She proclaims that Faraway Island is impossible to reach with the use of the S.S Tidal, but after Mr. Briney berates her for turning the player away, she reluctantly agrees. When the player arrives at Faraway Island, he/she will engage in a game of cat-and-mouse with Mew. After this, the player will be given a chance to battle and ultimately capture the Legendary Pokémon.
During the 2005 Pokémon Festa event, copies of the Old Sea Map were distributed to Japanese players, and has since never left Japan to be distributed elsewhere. As such, the only possible way for international players to acquire Mew internationally via the Old Sea Map is to use cheating devices, such as a GameShark or an Action Replay.