From Bulbapedia, the community-driven Pokémon encyclopedia.
Diglett's Cave (Japanese: ディグダのあな Digda's Hole) is a long tunnel dug by wild Diglett and Dugtrio between Route 2 south of Pewter City to the north and Route 11 on the eastern edge of Vermilion City to the south. The passage is famous for connecting Route 2 and Route 11, near Vermilion City. Regardless of what Generation a trainer encounters Diglett's Cave in, the trainer must pass through the cave at one point or another to either continue the storyline or to access the western Kanto municipalities of Pewter, Viridian, and Pallet.
Route description
As a whole, the cave runs from the northwest to the southeast, passing under Celadon City, Route 7, and Route 6 as it progresses toward Vermilion from Pewter. There are no points of interest along the cave; however, the cave is the only location to catch wild Diglett and Dugtrio in every Generation it has appeared in.
The usefulness of the cave as a connector route between Routes 2 and 11 has allowed Diglett's Cave to become one of the most important non-numbered routes in the Pokémon world. In fact, after the main character enters Cerulean City, the only way to return to Pewter City and Viridian City without using Surf or Fly is to travel through Diglett's Cave.
Items
Pokémon
Generation I
Generation II
Generation III
Differences Among Generations
In Generation II, the length of the cave was slightly less than it is in Generation I and Generation III.
References
Pewter City | 
Cerulean City | 
Vermilion City | 
Celadon City | 
Fuchsia City | 
Saffron City | 
Cinnabar Island | 
Viridian City
