Nuclear Holocaust

"One nuclear bomb can ruin your whole day." It was true back in the late 1950s, and it's just as true now. The world could end at any moment thanks to the countless nukes out there, just lying around. According to a 2010 Ploughshares Fund report, 22,000 active nuclear warheads are scattered around the globe. And that number doesn't include smaller—and therefore more susceptible to theft—nuclear weapons like atomic artillery shells and bunker busters. In truth, we're still exactly where we've been for the past 70 years: one international argument away from the end of the world.

Black Holes

Stars collapse on an almost daily basis. When a massive star collapses, it produces an immense gravitational pull along the way, drawing everything in its path—including light—toward its core, like a giant vacuum cleaner from which there's no escape. While black holes sit at the center of most galaxies, including our own, the real danger comes from a discovery made in the year 2000. "We now have conclusive evidence," physicist Michael Kaku told ABC News, "that there are wandering black holes—nomads, renegades—and right next to us in our own galaxy." How long until one of these nomads bumps into the Earth? No one is sure, and it's a big universe, but everyone agrees that it's going to happen sooner or later.

Nanotech

What if the programming of billions of tiny self-replicating robots—nanobots in the technical parlance—suddenly went awry? And instead of doing the job for which they were built (like mopping up an oil spill), they started consuming all matter on Earth—while building more of themselves? In his 1986 book, Engines of Creation, nanotech pioneer Eric Drexler describes it this way: "Imagine such a replicator floating in a bottle of chemicals, making copies of itself... the 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."

Gamma Rays

Gamma-ray bursts are high-energy beams of electromagnetic radiation shot out of a supernova, or exploding star. Researchers say a routine gamma-ray burst within 3,000 light years of the Earth would release more than enough radiation to completely cook our planet. Talk about global warming.

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