Pokémon Gold and Silver/Route 22
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Route 22 leads you back to the Pokémon League Reception Gate that you passed through earlier on your way to battle the Elite Four. If you talk to Professor Oak after you have all eight Kanto Badges, the person who was blocking the way to Mt. Silver will be gone.
You've been just about everywhere on the map, but your quest isn't over yet. Speak to Professor Oak, and he'll tell you of a location that isn't on the map: Template:~. You can get there from the crossroad between Template:~ and Template:~. To get back there quickly and easily, just head west from Template:~, and cut through Route 22. The Pokémon that you find here are pushovers, but they're about to get a lot harder...