Pokémon FireRed and LeafGreen/Diglett's Cave
From StrategyWiki, the video game walkthrough and strategy guide wiki
Diglett's cave, dug by Digletts, connects Route 11 and Route 2. There is no way you can get lost here and there aren't any trainers. However, you can catch Diglett and Dugtrio. When battling these two, Water, Grass and Ice Pokémon are great choices, but only Flying Pokémon can avoid their deadly Dig attack. So when the Pokémon digs, send out a bird. You will emerge north of Viridian City when you exit the cave.
