Author Peter Ward thinks he knows how the Earth will end. Well, perhaps not end, but cease being Earthlike -- the comfy place we know and love that supports mammalian life. By thinking across large spans of time (billions of years), Ward can extrapolate when Earth will cease to be habitable, due to natural planetary cycles. According to Ward's math, we're about halfway through the habitable stage of Earth -- but not to worry just yet: we've got many millions of years left.

Ward studies mass extinctions. While some come from asteroid or comet strikes, Ward thinks that some of the biggest extinctions come from the production of hydrogen sulfide, an extremely poisonous substance created by some bacteria. Ward's theory is an interesting look at the history of the world, and perhaps its future -- hear him explain his theory about hydrogen sulfide-related extinction (and its useful medical applications) in his TED Talk from February 2008: