From Bulbapedia, the community-driven Pokémon encyclopedia.
Faraway Island is a strange-shaped island located far away from Hoenn. Its location appears on an Old Sea Chart, and Captain Briney returns a favor by the ship in Lilycove City to sail to this far-off isle.
On the island is a sign with old, fading writing:
"…ber, 6th day If any human…sets foot here… again…et it be a kindhearted pers… …ith that hope, I depar…"
Mew is found on this island.