My money is on the grandmother and the wooden stick, I have seen this potent force in action and it is not one to be trifled with:) That said. Bacteria and Viruses have caused astounding, horrific damage to ecosystems (to say nothing of lots and lots of dead humans), in both the initial killing-offs and the subsequent after effects of the damage and panic they cause. And that's a mere gooey lifeform... not something that might operate outside of parameters dictated by biological requirements (ie, not killing all hosts as that gets you killed as well). Maybe not eat the world... unless we are talking about VERY sophisticated nanotech (or alien stuff... both of which EP appears to have), but think of the chaos that would be triggered in simply knowing that there was some sort of killing machine that you couldn't even see, in the water, in the air, maybe right next to you... right now. Size really does matter in this case. If you can quickly take out a vial or vaporize a base... OK... maybe problem solved. But if these buggers operate with half the tenacity of, say the bubonic plague or the common cold, in a few days (or less) it's got numbers that would require serious amounts of manpower to contend with. "Blue Goo" could work, but I wouldn't want to get in the middle of a nanotech brawl (let alone have it occur within...) Just my 2 cents, what say ye Rob and company? For what it's worth, check out Greg Bear's "Blood Music" and "Centruy Rain" by Allastor Reynolds. Both give good examples of the kind of catastrophic disaster that can occur with runaway nanotech, the latter doesn't feature the grey goo as much but details a situation where attempts to control the initial outbreak ultimately only escalated the damage as both nanobots "evolved" in a human-made artificial ecosystem.