Go to Jagged Pass and go in the middle where the Magma guy was. There is an earthquake and a door appears.
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go to mount pryre.
In Emerald, an old lady in Mt. Pyre will give you Magma Emblem. After you got it, go to Judged Pass. If you bring the Magma Emblem, the Magma Hideout will appear in Judged Pass.
Go to the top of Mt. Pyre summit and you will discover Maxie/Archie already took the Orb that was upon the summit. The elderly man will give you the Magma Emblem to open Team Magma's hideout.
the man on mnt. pyere gives it to you af ter you beat them on mnt. pyere
i don't know but i remember that you can access the Magma Cave/Hideout in order to battle Maxie / or even not. I think after you defeat Maxie, it is closed. But i Already Finished Pokemon Emerald..