The world of Phoenix is a very young world. It is a ball of phlogiston (the ethereal plane) with the elemental planes still fused to the prime material. In other words, in theory one could travel to the inner planes using mundane transport.
The world is basically that sphere of phlogiston, with mostly rock at the bottom, dividing the negative material from the light of the positive, while the above portion is mostly air, with the positive energy congregating at the very “top”. viewed from above, there is land bounded by endless seas on on side, and trackless deserts on the other, while in the center there exists a collage of all elements, forming a “mid world” or “middle kingdom” with features more like the worlds familiar to the Reborn.