Bubble Trouble: Golly! Ghost! 2/Secret Ending
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A special ending for the game can be accessed by eliminating every single enemy that emerges from Boss Water Ghost on the fifth scene (referred in-game as "FINAL SCENE"); once the scoreboard has shown the "ZIP" box being lifted higher than the "GHOST" box, the scoreboard will fade out and show the normal ending; and once the latter has finished, it will fade out and is instead replaced with a 16-bit version of that electro-mechanical diorama, as the game shows the end credits.
Once the credits have finished rolling, the text "THE END" will appear, and the high score table will appear; and once you have entered your initials, the game will go back into attract mode.