After beating Rogue Galaxy, you can take a trip to a Ghost Ship, a relatively small dungeon with some great treasures, but that's not all.

Upon completion of the Ghost Ship, go back to it, and go to the first area after getting off the Dorgenark. Go to the save point and you will see a man named 'Micon' standing opposite the big doubled-doors. Speak to him and he will ask you seven questions. Answer them correctly, and he will give you the 'Key to the Underworld'.

If you answer all seven questions correctly, you'll get the key, Micon disappears, and a question mark will replace him. Step into the question mark, and use the key to access the Ghost Ship Extreme. This is a 100 level dungeon.