An often cited worst-case scenario is the so-called "grey goo", a substance into which the surface objects of the earth might be transformed by amok-running, self-replicating nano-robots, a process which has been termed global ecophagy. Defenders point out that smaller objects are more susceptible to damage from radiation and heat (due to greater surface area-to-volume ratios): nanomachines would quickly fail when exposed to harsh climates. More realistic are criticisms that point to the potential toxicity of new classes of nanosubstances that could adversely affect the stability of cell walls or disturb the immune system when inhaled or digested. Objective risk assessment can profit from the bulk of experience with long-known microscopic materials like carbon soot or asbestos fibres.
As originally coined and first defined by Robert Freitas, the term ecophagy means, literally, the consuming of an ecosystem. Thus global ecophagy is the consumption of the entire worldwide ecosphere, for instance, by out-of-control self-replicating nanorobots. This is one well-known scenario for a technological singularity. However, the word ecophagy has now been adopted by others concerned about less specific threats to the biosphere. There are other forms of planetary ecocide, and even intentional ecophagy — for instance, there was a long-standing USENET group called "alt.pave-the-earth" devoted to paving the entire Earth, including the oceans, with asphalt or concrete. More seriously, those with concerns about global economic monoculture as a threat to biosafety and biosecurity have used the term to refer to the potential for total loss of natural capital to economic processes, involving robots or just very stupid humans and institutions. The more common term for this is Easter Island Syndrome, which refers to the deforestation of that island by its own inhabitants in the 1500s, before any contact with Europeans. The paper Some Limits to Global Ecophagy by Biovorous Nanoreplicators, with Public Policy Recommendations was published in April 2000 by Freitas, where he wrote:
Grey goo refers, usually in a science fictional context, to a hypothetical end-of-the-world event involving nanotechnology, in which out-of-control self-replicating robots consume all life on Earth while building more of themselves (a scenario known as ecophagy). In a worst-case scenario, all of the matter in the Galaxy could be turned into goo (with "goo" meaning a large mass of replicating nanomachines lacking large-scale structure, which may or may not actually appear goo-like), killing the Galaxy's residents. The disaster could result from an accidental mutation in a self-replicating nanomachine used for other purposes, or possibly from a deliberate doomsday device. It is unclear whether nanotechnology is capable of creating grey goo at all. While the biological matter that composes life releases significant amounts of energy when oxidised, this energy might not be sufficient for the robots to continue replicating. Some also consider it unlikely that an artificial self-replicator could spontaneously evolve in a manner that could present an immediate threat.
Assuming a nanotechnological replicator is capable of causing a grey goo disaster, safety precautions might include programming them to stop reproducing after a certain number of generations (but see cancer), designing them to require a rare material that would be sprayed on the construction site before their release, or requiring constant direct control from an external computer.