Defeat all the Kanto gym leaders. While you are able to do most of them in any order, Blue can only be fought if you have the other 7 Kanto gym badges.
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outside of the power plant in Kanto after you have goten all 16 gym badges.
You can access Cerulean Cave in SoulSilver after you get all 16 gym badges (Johto and Kanto).
Get all eight badges from the johto region and then beat the Pokemon league to go to kanto
To get into Cerulean Cave you have to get all 16 badges in Kanto & Johto, beat Red in mt Silver, then talk to Prof. Oak
You need to get all 16 badges and defeat red at the top of mt. silver. Then professor oak will call you.