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This place is inside the crater of a volcano. Obviously, most of the Pokémon here are fire type. At the end of the Tunnel where you find all Magnemite, look right and you'll meet an Electrode. Hit it with an apple to make it explode, opening up a new passage.

• Pokémon: Charmander, Charmeleon, Charizard, Vulpix, Growlithe, Arcanine, Rapidash, Magmar, Magikarp, Moltres
• Pokémon Sign: Koffing Smoke