Unlockable stage

An unlockable stage is a stage that is not available at the start of the game (unlike a starter stage), but must be unlocked. Like an unlockable character, it is required to accomplish a particular in-game feat and once completed, a message will be shown, telling the player that the stage has been unlocked.

In Super Smash Flash 2
For the final version, any information is unknown.

In demos
These stages were made unlockable when first introduced in their respective demo versions: