Magic the Gathering (MTG) - Darksteel Reactor

This is one of my favorite MTG pieces of all time. Originally titled "Impervium Doomsday Reactor", it is one of the most impressive pieces of darksteel on Mirrodin (With the possible exception of Darksteel Colossus) but it wins out for me because of how it seems to work: by trapping and concentrating the golden rings and blobs of magical energy that characteristically surround fully formed darksteel, and confining them all in the center of an indestructible inner chamber until the magical energy reaches a critical mass . . . Since darksteel has to be fully formed as it it created, being able to predict how this energy would flow around the piece before you make it, and the fact that it's at least a few miles high, makes it probably one of the most interresting things on Mirrodin.