Amazon | Nano-machines stand poised to revolutionize technology and medicine, but what happens if these minuscule beasties break their leash and run amok? Rees, the U.K.’s Astronomer Royal and prolific author (Just Six Numbers; Our Cosmic Habitat), warns that the 21st century may well witness the extinction of mankind, a doomsday more likely to be caused by human error than by a natural catastrophe. Bioterrorists are the most widely publicized threat at the moment, but well-intentioned scientists, Rees says, are capable of accidentally wiping out mankind via genetically engineered superpathogens that create unprecedented pandemics, or even through something as weird as high-energy particle experiments that backfire and cause the universe to implode. Rees poses some hard questions about scientists’ responsibility to forsake research that might lead to a malevolent genie being let out of its bottle and even to restrict the sharing of scientific information to prevent it from getting into the wrong hands. Ultimately, though, Rees sounds more alarmist than precautionary. Some may find him overly optimistic on what science will be capable of doing in the next quarter century. Rees makes some provocative points, but the book falls short of what readers expect from a scientist of his stature.