Among the cognoscenti of nanotechnology, this threat has becomeknown as the “gray goo problem.” Though masses of uncon-trolled replicators need not be gray or gooey, the term “gray goo”emphasizes that replicators able to obliterate life might be less in-spiring than a single species of crabgrass. They might be superiorin an evolutionary sense, but this need not make them valuable.