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Iron Island is where you'll get a rare egg so you need an empty slot in your party. No HMs are needed. Exit your boat and climb the stairs into the cave.

You can get some good experience leveling up. You will be attacked by a lot of Graveler and that's new to your Pokédex. Ice and Water attacks will make short work of the enemies. There are also some trainers and items found here.

Note also that you will have a companion who will heal you after every battle, so feel free to battle wild Pokémon as much as you want.

[edit] Getting through Iron island

Here's what to do for clearing the cave:

• Go down the eastern flight of stairs.
• Climb upstairs, then east, then downstairs, then south, then up the eastern of stairs, down the elevator.
• Go down west set of stairs, get partner.
• Take the first path south, fight some trainers, then continue south, then upstairs, then southward to fight a handful of trainers, down stairs, up stairs,beet team galactic and down the elevator.
• This gets you the Riolu egg. Go through the cave door, up the elevator, and out of the cave.

[edit] Pokémon Encounters

Name	Type	Notes
Graveler	Ground/Rock	Normal Encounters
Onix	Ground/Rock	Normal Encounters
Steelix	Ground/Steel	Normal Encounters
Riolu	Fighting	Hatched from the egg received from Riley at the end of the cave. Evolves into a very powerful Pokémon at level 30 but if it doesn't treat with care.
Geodude	Ground	Rare Encounters
Zubat	Poison/Flying	Rare Encounters
Riolu Egg	Egg	Get it from Riley after you beat team galactic
Golbat	Poison/Flying	Normal Encounters