This peculiar dimension was formerly inhabited by Kassil before Cheshire Cat decided to wander in; the sudden presence of another lifeform in the pantemporal dimensional matrix caused it to collapse, expelling both present within it; CC was unharmed, not being a natural resident, while Kassil's many potential selves were expelled randomly across all the timelines he existed in. The one who ended up on the Island was a somewhat confused engineering of steam and clockwork, capable of creating clockwork creatures that assumed a life in and of themselves.1)

In theory, four-space still exists at the Planck Scale, but it is effectively out of the reach of even the wildest Mad Science or Improbable Effect.2)3)

1)
Such as Fish, the clockwork flying fish he created for Skidge.
2)
We hope.
3)
Recent events suggest that it may not, in fact, be beyond reach, with Kassil's return to the Island and his cryptic comments about 'fixing' an accident.