Originally the pet project of some unnamed planeswalker, this plane of existance has nine spheres, each nested within each other.
The first sphere started out as a fairly normal land, very large with varying wildlife. The creator of the world was fond of dragons, so there are wurms and such around. The difference was that the sky did not change from its "glow" of darkness. The wurms were metal, little more than sentient machines.
The second sphere is what is poetically referred to as the foundations of the world. A vast and cavernous place, it is full of the struts and columns necessary to support the first sphere. Also, the creatures on the first sphere originated here, as small mechanical beasts.
The third sphere are where the furnaces are, miles high and volumnous.
The fourth sphere is the location of the Fane of Flesh, where all the creatures of Phyrexia begin. Most start out as vat-grown newts, sexless and identacle creatures. When a newt has shown itself to be worthy of it, it is compleated, which means part or all of its flesh is replaced with machinery and metal.
I'll have to retrieve my book for the next four spheres...
The nineth sphere is the seat of power. It is small, only about the size of a large laboratory (maybe 20m x 20m). Before the arrival of Yawgmoth, this area was also filled with the corpse of the former owner.
Yawgmoth claimed the plane, and began setting it up as a paradise for the Thran. His idea of paradise didn't quite match up with that of others, however, and thus began the whole story behind the Magic: The Gathering card game.