Walkthrough
Within Route 11 is Diglett's Cave, a small cave that connects back to the previously inaccessible portion of Route 2. Only two different Wild Pokemon can be found in Diglett's Cave, making there not much to it. The main goal at this point is getting back to Route 2 to meet up with Professor Oak's Aide. If you meet his Pokedex Requirement, he will gift you with HM Flash. This HM will be essential to making it through Rock Tunnel a little later on. After obtaining HM Flash, head back to Cerulean City and receive your free Bike via the Bike Voucher. After you do, head east of Cerulean City (with the help of HM Cut) on Route 9.
Wild Pokemon
Walking:
Diglett (95%): Levels 15-22
Dugtrio (5%): Levels 29-31