Use surf under the bike thing in mauville and you'll c a cavelike thingy and go in it with the key and in there is kind of a maze with magnet pokemon. the end..
In Pokemon Emerald, New Mauville is south of Mauville City. Go to the first Fisherman you see when heading south from Mauville City. Surf on the body of water that he is facing, then turn left. As...