Surf to get there and Rock Smash inside for the maze (Fire Red)
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yes you need all 16 badges to get into cerulean cave.
there is no cerulean cave in ruby
Unknown Dungeon was the name of a cave in Red, Blue and Yellow version that is located northwest of Cerulean City you would need Surf to reach their the cave is now called Cerulean Cave.
YoU need to get all 8 gym badges in kanto
You can access Cerulean Cave in SoulSilver after you get all 16 gym badges (Johto and Kanto).