Phyrexia
An artificial plane of entirely mechanical "life" created by an ancient planeswalker. Little is known of this planewalker, aside from the fact that he enjoyed assuming the form of a dragon. This world was not so different from Mirrodin until Yawgmoth arrived. Yawgmoth, and the remnants and descendants of the phthisis-inflicted Thran whom he "saved", lived on this plane.
Structure of Phyrexia
The plane consisted of nine nested spheres, each with its own purpose and, often, mechanical ecosystem.
First Sphere
First Sphere of Phyrexia is a mechanical parody of life. It's covered by artificial jungle bedewed by oily rain. It's home of few forms of dragon engines and many others mechanical animals simulating true life.
Second Sphere
Third Sphere
Third Sphere is filled with metal pipes creating space ditortions what prevents from planeswalking to lower spheres.
Fourth Sphere
Fifth Sphere
Fifth Sphere is the ocean of glistening oil used by Vat Priests of Fourth to breed newts.