Hi,
My opinion:
Firstly, if it is true about this machine being 3 blocks long and 3 stories high....I would call it a "Beast" alright.
Secondly, "It controls everything"...hmmm...thats a possiblility if you consider the extend of the computer networking systems around the world. eg, With your Visa card, u can actually be "tracked" whenever you make a purchase anywhere in the world.
Finally, for the end off days......I believe it will happen one day...but when, I dare not speculate.
In short: I believe in the possibility of such a machine existing. Where it is located? I dont know.

Cheers
Ps,: mainframes can be really huge!...I work in a mainframe environment...I know..Trust me.