The Magma Hideout is one of the new Areas discovered in Pokémon Emerald situated in at the base of Mt. Chimney. You need to go into it to further the game

You can enter it as soon as you have the Magma Symbol from Mt. Pyre. This Hideout is a cave with long winding tunnels and many Magma Members to battle
It is big, has many Wild Pokémon and at the end is a Big Suprise
You find that Maxie has Groudon here in it's Sleep and as you arrive he awakens it for Groudon to disappear. Magma soon escape the Hideout

Wild Pokémon in Magma Hideout

Here are the Pokémon you will find in the wild in Magma Hideout:

In The Tunnel
	Torkoal	Common
	Geodude	Uncommon
	Graveler	Rare
	Slugma	Rare