There are many secret stages that can be unlocked in Brawl. The following is a complete list of them and how to unlock them.
75m: Use Donkey Kong in Vs. Mode battles 20 times
Big Blue: Use Captain Falcon in Vs. Mode battles 10 times
Electroplankton: Complete Event # 28
Flat Zone 2: Unlock Mr. Game & Watch
Great Sea: Unlock Toon Link
Green Greens: Use Kirby in Vs. Mode battles 20 times
Green Hill Zone: Unlock Sonic
Jungle Japes: Play on any Melee Stage in Vs. Mode battles 10 times
Luigi's Mansion: Use Luigi in Vs. Mode battles 3 times
Mario Bros: Complete Event #19
Pokemon Stadium: Play Pokemon Stadium 2 in Vs. Mode battles 10 times
Spear Pillar: Complete Event #25
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