https://en.wikipedia.org/wiki/K._Eric_Drexler
Eric Drexler famously argued in his 1986 book, Engines of Creation, that if nanoscale machines (assemblers) could build materials molecule by molecule, then using billions of these assemblers, one could build any material or object one could imagine. But in order to get that scale, scientists would have to build the first few nano-assemblers in a lab and direct them to build other assemblers, which would in turn build more, growing exponentially with each generation.
Drexler worried that such a situation could, however, grow quickly out of control as assemblers began to convert all organic matter around them into the next generation of nanomachines in a process he famously called the “gray goo scenario,” one in which the earth might be reduced to a lifeless mass overrun by nanomachines. How might such a doomsday scenario play out? Let’s say in the future billions of nano-bots were released to clean up an oil spill disaster in an ocean. Sounds great, except that a minor programming error might lead the nano-bots to consume all carbon-based objects (fish, plants, plankton, coral reefs) instead of just the hydrocarbons in the oil. The nano-bots might consume everything in their path, “turning the planet to dust.” To understand just how quickly this might happen, consider the example Drexler provides in his book:
Imagine such a replicator floating in a bottle of chemicals, making copies of itself…[T]he first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined—if the bottle of chemicals hadn’t run dry long before.
Goodman, Marc. Future Crimes (p. 437). Knopf Doubleday Publishing Group. 2015, Kindle edition.