The Pokémon Emerald Old Sea Chart was an event item released in Japan in 2004 and 2005, and was subsequently made unavailable. The chart allowed players to find Faraway Island, which was a required step to capture the Pokémon Mew.
Once a player has the Old Sea Chart, also called the Old Sea Map, characters in Pokémon Emerald offer to take him to Faraway Island, where he can find Mew. The only way to get an Old Sea Chart now is by merging records with a Pokémon Emerald player who already has it. There are also several Gameshark codes that allow a player to cheat his way to Faraway Island.