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What Could Cause Our Earth to End?

We've had a good run at it. Over the past 500,000 years, humans have roamed the land, we've built cities, created complex languages, and sent robotic scouts to other planets. But all good things must come to an end. One day Earth will be inhospitable to anything resembling life as we know it.

It's difficult to imagine it all coming to an end. Yet 99 percent of all species that ever lived have gone extinct, including every one of our hominid ancestors.

The life on this planet likely won't cease until billions of years from now. But, depending on the fluctuation of astrophysics, it could also happen tomorrow or anytime in between.

Here are the many ways scientists believe Earth could die:

• Asteroid impact
  -An asteroid five miles wide would cause major extinctions, like the one that may have marked the end of the age of dinosaurs. For a real chill, look to the Kuiper belt, a zone just beyond Neptune that contains roughly 100,000 ice-balls more than 50 miles in diameter. The Kuiper belt sends a steady rain of small comets earthward. If one of the big ones headed right for us, that would be it for pretty much all higher forms of life, even cockroaches.
• Gamma-ray burst
  -Earth's atmosphere would initially protect us from most of the burst's deadly X rays and gamma rays, but at a cost. The potent radiation would cook the atmosphere, creating nitrogen oxides that would destroy the ozone layer. Without the ozone layer, ultraviolet rays from the sun would reach the surface at nearly full force, causing skin cancer and, more seriously, killing off the tiny photosynthetic plankton in the ocean that provide oxygen to the atmosphere and bolster the bottom of the food chain. All the gamma-ray bursts observed so far have been extremely distant, which implies the events are rare. Scientists understand so little about these explosions, however, that it's difficult to estimate the likelihood of one detonating in our galactic neighborhood.
• Rogue black holes
  -A black hole wouldn't have to come all that close to Earth to bring ruin; just passing through the solar system would distort all of the planets' orbits. Earth might get drawn into an elliptical path that would cause extreme climate swings, or it might be ejected from the solar system and go hurtling to a frigid fate in deep space.
• Supervolcanoes
  -The most inexorable threat to our modern civilization, however, is homegrown--and it strikes much more often than big cosmic impacts do. Every 100,000 years or so, somewhere on Earth, a caldera up to 50 kilometers in diameter collapses and violently expels heaps of accumulated magma. The resulting supervolcano is both unstoppable and ferociously destructive. One such monster, the massive eruption of Mount Toba in Indonesia 74,000 years ago, may have wiped out most humans on Earth, causing a genetic bottleneck still apparent in our DNA--although the idea is controversial.