Phyrexia

An artificial plane of entirely mechanical "life" created by an ancient planeswalker. This world was not too different than Mirrodin until Yawgmoth arrived. Yawgmoth, and the powerstone-diesease-infected 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.