The 9th Dimension was originally going to be in Crash Twinsanity but from the final version, it was cut due to time. It is unknown, but probably the Sea Level would be located somewhere here.

There is also a little development 'joke' which Cortex tells upon getting back to the Iceberg Lab from the Academy of Evil: "It was going to be 2 dimensions but we ran out of time". Telling us that the game got rushed and it was really going to be two dimensions, but the developers ran out of time and the content were never added.