To pass here you will need the moves SURF and STRENGTH, all the skills, and best Pokémon you have. There are TOUGH Pokémon here (Level 40
Marowaks, Level 40 Venonats, etc.) and very skilled trainers. This is where you will also find Moltres, the Legendary Fire Bird. There are a lot of puzzles, and by the end you will probably be VERY worn-out, but still wondering how come Gary was again one step ahead of you, as explained by the people in the cave.
