Grey Goo is like a Horde of Alien Locusts, only replace Alien Locusts with Nanotechnology -- and instead of grass, they "eat" anything. Or, if the protagonists are lucky, just anything mineral or electronic. For some reason, most of the time these specifically were created to clean up oil spills. Can cause The End of the World as We Know It -- specifically, Type 3b or 4 on Apocalypse How. Compare Blob Monster. Examples of Grey Goo include: