First off the grey goo theory sounds like a rip off of a REAL theory. Think about a tiny machine that has a blueprint for itself in its memory storage matrix. The ability to reproduce itself.



Now give that little nano machine the fucntion of catalyzing carbon in our air and turning it into graphite.

Basically the entire surface of our planet would be covered by a mile high solid crust of graphite (pencil lead) within a few months. It could all be started with one single microscopic nanomachine.