use cheats
you go to an island and it is there. i think it is the 8th island.
there is no mushroom island.
go to cinnabar island and once you beat the Pokemon mansion Bill will come out and offer you to go with him to first island.
Cinnabar island
in pokemon emerald you can go to the two island next to the battle frontier
beat the pokemon leage and go to snowpoint and take the boat to the island catch heatran and go back to the island and...TADA its there
on Cinnabar Island.Pokemon mansion is on cinnabar island if you don't know how to get there go to pallet town then go south surfing on one of your Pokemon and there your at cinnabar island.
in the secreat cave at cerulean city along with mewtwo zaptos and artacuno you need to have beaten the elete four and have a Pokemon that knows surf go past the nugget bridge to your left surf the water all the way until you see it
you have to go to the dark crater floor 6 with a secreat slab and a mystery part and here are there wondermails secreat slab 3NYW PYSH S#RX 5=Y8 O2T# K526 mystery part =NNR+5+NFR1 +NQR7H-W%4@9P there i hope this helped :D
there will be one island with loads of dissapearing trees and a few rocks go through and go on a small island and go to the middle you will go down and you will be there
go to a Pokemon event
You have to have a cheat. (For example a game shark).Or you have to go to a Pokemon convention and they will give you a ticket to the island.