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A variant on the gray goo nanotechnology disaster scenario. In this case, a type of nanobots designed to make diamonds out of carbon dioxide (diamonds being used now for all sorts of applications because they are cheap) goes haywire and begins to convert all the earths carbon dioxide to O2 and diamond. As the nanobots spread, plants cannot photosynthesize CO2 and water into glucose. Also, the greenhouse effect is destroyed, but down to the level where the earth gets really really cold. The plants die out, but the nanobots now create O2 from our CO2, so we can still breath. We now have to become dependant on nanobots to synthesize food for us and the animals, because plants are dead. This creates a scenario where our entire existance is based on nanobots, which would be a precarious situation.