Mt. Silver, Johto

Mt. Silver is a mountain that has many rare pokémon. The problem is, it's prohibited to the regular traveler because these pokémon are so strong, it's scary and dangerous! Only trainers who have obtained Professor Oak's special permission from are allowed to proceed towards this problematic destination.

Many trainers attempt to come to Mt. Silver to complete their Pokédex

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There's a cave entrance to Mt. Silver where a big confusing cave with many doors is. Red was training inside this cave in the year 2000. A trainer from New Bark Town fought with him in a pokémon battle (the toughest one he'd ever had) and defeated him! It can not be expected that Red's level 80 pokémon were easy to beat, though.