Part 12: Beat the Elite Four and the Champion

Beat the Elite Four and the Champion opens 1 area.

Where to go

1. Cerulean Cave

   Wild: Magikarp 100% · Lv 5, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10, Kingler 25% · Lv 23 +20 more

Everything you can have caught by this checkpoint · Open the interactive guide

It also doesn't say when Kanto Underground Path opens up, so it is in no part above; it has its own route page.