From the introduction:
The eight previous worlds have left the Earth awash in the transdimensional. Devices, prior-world structures, and even many of the creatures featured in the Numenera corebook, like the abykos and the Nibovian wife, are entities that exist partially out of phase or hail entirely from another plane. At least one of the prior worlds explored and harnessed alternate dimensions as a means for increasing their own power and knowledge. Many of these entrances and exits still remain.
INTO THE OUTSIDE
Let’s Travel the Multiverse!
The Ninth World is filled with the bizarre and unexpected. But beyond it, the truly astonishing awaits. Enter a dimension where a traveler exist in three different places simultaneously. A pocket universe where an explorer is transformed into living sound. An existence where time carves living beings into separate time streams. Or a world where merely thinking changes reality. How can an adventurer even begin to cope with such radically different worlds? Join us and see.
What will you find when you step into the outside?
Ten transdimensional locations. Visit the Banded Bluff, the Endless Abode, the Whispering World, Celerillion, or mysterious Reeval—the source of the Nibovian threat.
Multiple modes of dimensional travel. Explore the Sideslip Fields. Pilot the Crystal Ship. Take a ride on the Silent Nyek. Or enter the Grinder of Infinities.
New creatures, including the arravelon, the stealthy pariall, the carnivorous color, the time wraith, and the deadly lycidaris, a dimensional locust.
Twelve alternate realities, from the Ascension of Kex-Lianish—a world consumed by living crystal—to the Bloom, a vast creature composed of maws that can provide transport you to other dimensions.