Grey Goo

Nanotechnology of molecular level structures fabricated by mankind for some purpose results in very small devices that would be required in large quantities for practical purposes. One might grow them rather than build them, giving the structures the ability to self-replicate. If the structures escape control and if they can convert any matter into themselves, the sequence of events could cause ruin. As they build themselves they destroy the Earth itself, creating a uniformity of itself rather than the immense diversity of life itself. This theoretical result is called grey goo.
Grey goo is a catastrophic theory suited for science fiction, but it is straight out of the works of molecular nanotechnology engineer K. Eric Drexler. Drexler later published a paper renouncing grey goo. Wired magazine published an article describing a scenario where biotechnology at the molecular level creates a green goo. People focused on nanotechnology have given this a lot of attention too, and despite the scary prospects it appears we are all safe from the goo.