The authors evaluate the risks from several popular doomsday scenarios including smallpox biological terrorism, grey goo, and nuclear terrorism.

The far-fetched theory that nanotechnology could lead to a 'grey goo' catastrophe has captured the public imagination despite being discredited by scientists.

First it was "gray goo," the threat of self-replicating machines populating the planet. Now an environmental think tank is raising the specter of "green goo," where biology is used to create new materials and new artificial life forms.

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Unleashed viruses, environmental disaster, gray goo--astronomer Sir Martin Rees calculates that civilization has only a 50-50 chance of making it to the 22nd century.

The scientist many regard as the father of nanotechnology has backed away from his famous claim that nanomachines could turn the planet into "grey goo".

Eric Drexler argues that self-replicating machines are not vital for large-scale molecular manufacture, and that nanotechnology-based fabrication can be thoroughly non-biological and inherently safe.

An exchange with Bill Joy on the risks of a catastrophic "civilization-changing event" from misuse of nanotechnology, genetic engineering, or artificial intelligence.

Environmentalists and nanotechnology advocates are squaring off over the promise and perils of nanotechnology.

Gregg Easterbrook offers a skeptical guide to the most common doomsday scenarios.

The author warns that excessive precaution could strangle nanotechnology at birth, arguing that "it is doubly absurd to be anticipating catastrophe, especially when based on implausible claims about machines that nobody can make doing things that defy the rules governing the physical world."