Rock Tunnel is a passageway in the Kanto Region from Cerulean City to Lavender Town. In the Generation I games, it is the only way to progress at the time required to take it, due to a sleeping Snorlax on Route 12. Because of the dark, a Pokémon who can learn Flash is required to go into rock tunnel. The only way to learn flash is by receiving an HM05 from Prof. Oak's Aide near Diglett's cave.
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