Chaos
Finish the Encore Extra Stage with a full-combo, and you'll unlock chaos, which according to some is how nature itself has propagated across this tiny planetoid of ours.

Chaos #2
If the above doesn't work for you, try this instead: If you obtain a grade 'AA' (9,500,000+ points) in the Expert difficulty on the final stage of your credit, you will advance to the Extra Stage. This does not work on the Tutorial, Nonstop or Challenge modes, or on Battle mode. In the Extra stage, you must play on expert difficulty with these options: 1.5x, Reverse, Rainbow, and Pressure Gauge, which makes your lifebar start at full but also prevents you from recovering energy.

Depending on the difficulty you chose, you may be presented with a "boss" song to play for your Extra Stage. If you successfully pass this song, you may then play the 'One More Extra Stage'. For this, you must play the hidden song "Chaos" on Expert difficulty, with no options, and on Sudden Death, where you fail instantly if you obtain even a single judgement of Good, Almost, Boo or N.G. Easy mode has no boss song, so you cannot access the OMES on this mode.

Chaos #3
Boss Song: Chaos: Choose and clear Healing-D-Vision or Fascination MAXX as your Extra Stage
Boss Song: Fascination: Choose and clear Eternal Love Mix - Access Extra Stage on Difficult or All Music mode (must be on machine with sync update).
Boss Song: Fascination MAXX: Access Extra Stage on Difficult or All Music mode
Boss Song: Healing-D-Vision: Access Extra Stage on Medium mode

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