When you defeat the elite four now you will go to the cave in Cerulean City.
By dying
You can go there after beating the elite four.
you go on to the battle frontier and earn symboles
You can go to Kanto and beat the Gym Leaders and Elite Four there. You can also get Kanto and Hoen starter Pokemon.
Prof oak after beating the elite four.
go get mewtwo, the legendary dogs go to 1,2,3,4,5,6,7 island and have fun
Go there after beating the Elite Four. The ship is in Snowpoinnt City.
you can go on the seivii islands 4 to 7
go to cerulean cave and catch mewtwo
Go to Evergrande City to beat the Elite Four
After beating the Elite Four talk to Celio, the man on the first island.
To face the elite four at indigo plateu