After you have found the Old Sea Map on Route 22, take it to the old captain in Sunset City. He can take you to this remote island.
Pokémon Lab Edit
Hidden in the grass is the extremely coveted Mythical Pokémon: Mew.
Obtainable Pokémon Edit
|Mew||30||Hiding in the grass.|
Obtainable Items Edit
|Comet Shard||Item Ball near the entrance.|
The Old Sea Map was initially intended to be a limited time Mystery Gift, but due to technical issues, was made into a regularly obtainable item.