I saw it on Discovery too - damn at least we all watch educational stuff
It has to be so big because it is used for ore extraction - it strips ore like there is no tomorrow! I also think it has its own power plant inside of it, and it uses electricity to power itself. They would have to have tanker trucks of fuel following it just so it would make it 1 mile if it use fuel ;)
Mike
They say even with the cost of the thing that it still pays for itself in the work it does. I think this is the one they ship in pieces to your site, then assemble it on site. It takes 1.5-2 years just to put it together! ( If I remember correctly from Discovery )