Ah the so-called Grey Goo Scenario. Little molecular nanorobots tearing apart the Earth molecule by molecule, replicating themselves until the entire Earth is covered with nothing but a sludge of nano 'bots. Of course, this scenario is highly unlikely. These things require energy to work, and energy is not freely available. Freeze them, and they stop working; heat them up enough and they're permanently destroyed They would, therefore, be fairly easy to control. I've got more important things to worry about. Heh.