new info from a faq at gamefaqs,
Mew
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After a few months of Mudkips games, I have finally been able to crack the
mew mystery, and here it is:
Conditions:
-Get both Ruby and Sapphire Plates and have access to the unknown dungeon
-Unknowns !?
-Mewtwo
-Ditto
Once all conditions are done, put Mewtwo in your first slot, and have ditto in the second, with Unknown ! being third, and Unknown ? being fourth.Have a Pokemon that knows surf, and fly to island 6 and swim to the transfermation cave after the forest. Once you enter, the ground will start shaking and reveal a flight of stairs. Continue until you get to the final room, where mew is. He is at level 30, so be careful.