Destructive nanobots . opposite of Blue Goo. See Star Trek scenario. Vast legions of destructive nanites. Typically, created by accident. Left unchecked, they will basically convert everthing they contact into more of themselves, or consume and digest it for energy. Either way, its pretty much bad news. The debate rages on. cheque out the first technical analysis of grey goo ever published, in April 2000, by Robert A. Freitas Jr. Also - Self-replicating (von Neumann) nanomachines spreading uncontrolably, building copies of themselves using all available material. This is a commonly mentioned nanotechnology disaster scenario, although it is rather unlikely due to energy constraints and elemental abundances. More probable disaster scenarios are the green goo, golden goo, red goo, khaki goo scenarios. As a protection blue goo has been proposed.