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.