While at work I saw this story. Self-replicating machines, aka von Neumann machines, are the #9 method (scroll down a bit) of destroying the earth with a feasablity rating of 8/10. Lucky these robots can only make copies of themselves out of Legos or something, and don't work at a small enough scale that they'd threaten to overwhelm the planet. But there's always a first step and technology always gets smaller with time. Soon we'll have weird flying arms decending upon our cities for their readily plundered materials, and then the rest of the planet will be turned into these automatons of doom. Sorry, that should read: "automatons of DOOOOOOOOOOOM!!!" Get the EMP generator ready; hope it works.