Charmander-Squirtle, you have to go to solar cave and find a key on floors 1-10 and then on the 10th lvl there is a passage that can only be opened with a key. You must also know the IQ move "All Terrain Hiker," because it is surrounded by a moat. Unless you are a Flying, Dragon, or Water type guy. You might also get a link cable (evolutions{Machoke-machamp, haunter-gengar, graveler-golem, kadabra-alakazam) or the HM SURF, WATERFALL, DIVE, or FLY
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Be sure to teach Surf, dive and waterfall to a water pokemon you take with you every mission (Squirtle-Wartorle-Blastoise, etc.)